diff --git a/newsfragments/3467.minor b/newsfragments/3467.minor
new file mode 100644
index 000000000..e69de29bb
diff --git a/newsfragments/3473.minor b/newsfragments/3473.minor
new file mode 100644
index 000000000..e69de29bb
diff --git a/src/allmydata/mutable/filenode.py b/src/allmydata/mutable/filenode.py
index 849dc4c88..6f96cabf4 100644
--- a/src/allmydata/mutable/filenode.py
+++ b/src/allmydata/mutable/filenode.py
@@ -147,9 +147,9 @@ class MutableFileNode(object):
 
     def _get_initial_contents(self, contents):
         if contents is None:
-            return MutableData("")
+            return MutableData(b"")
 
-        if isinstance(contents, str):
+        if isinstance(contents, bytes):
             return MutableData(contents)
 
         if IMutableUploadable.providedBy(contents):
@@ -884,9 +884,9 @@ class MutableFileVersion(object):
         d = self._try_to_download_data()
         def _apply(old_contents):
             new_contents = modifier(old_contents, self._servermap, first_time)
-            precondition((isinstance(new_contents, str) or
+            precondition((isinstance(new_contents, bytes) or
                           new_contents is None),
-                         "Modifier function must return a string "
+                         "Modifier function must return bytes "
                          "or None")
 
             if new_contents is None or new_contents == old_contents:
@@ -960,7 +960,7 @@ class MutableFileVersion(object):
         c = consumer.MemoryConsumer()
         # modify will almost certainly write, so we need the privkey.
         d = self._read(c, fetch_privkey=True)
-        d.addCallback(lambda mc: "".join(mc.chunks))
+        d.addCallback(lambda mc: b"".join(mc.chunks))
         return d
 
 
@@ -1076,7 +1076,7 @@ class MutableFileVersion(object):
             start = offset
             rest = offset + data.get_size()
             new = old[:start]
-            new += "".join(data.read(data.get_size()))
+            new += b"".join(data.read(data.get_size()))
             new += old[rest:]
             return new
         return self._modify(m, None)
diff --git a/src/allmydata/mutable/publish.py b/src/allmydata/mutable/publish.py
index d4b6f4dea..8816855e7 100644
--- a/src/allmydata/mutable/publish.py
+++ b/src/allmydata/mutable/publish.py
@@ -305,7 +305,7 @@ class Publish(object):
         # Our update process fetched these for us. We need to update
         # them in place as publishing happens.
         self.blockhashes = {} # (shnum, [blochashes])
-        for (i, bht) in blockhashes.items():
+        for (i, bht) in list(blockhashes.items()):
             # We need to extract the leaves from our old hash tree.
             old_segcount = mathutil.div_ceil(version[4],
                                              version[3])
@@ -794,7 +794,7 @@ class Publish(object):
     def push_blockhashes(self):
         self.sharehash_leaves = [None] * len(self.blockhashes)
         self._status.set_status("Building and pushing block hash tree")
-        for shnum, blockhashes in self.blockhashes.items():
+        for shnum, blockhashes in list(self.blockhashes.items()):
             t = hashtree.HashTree(blockhashes)
             self.blockhashes[shnum] = list(t)
             # set the leaf for future use.
@@ -867,7 +867,7 @@ class Publish(object):
         ds = []
         verification_key = rsa.der_string_from_verifying_key(self._pubkey)
 
-        for (shnum, writers) in self.writers.copy().items():
+        for (shnum, writers) in list(self.writers.copy().items()):
             for writer in writers:
                 writer.put_verification_key(verification_key)
                 self.num_outstanding += 1
diff --git a/src/allmydata/test/mutable/test_datahandle.py b/src/allmydata/test/mutable/test_datahandle.py
index 39d65557d..1819cba01 100644
--- a/src/allmydata/test/mutable/test_datahandle.py
+++ b/src/allmydata/test/mutable/test_datahandle.py
@@ -1,17 +1,29 @@
+"""
+Ported to Python 3.
+"""
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
+from future.utils import PY2
+if PY2:
+    from future.builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, list, object, range, str, max, min  # noqa: F401
+
 from twisted.trial import unittest
 from allmydata.mutable.publish import MutableData
 
 class DataHandle(unittest.TestCase):
     def setUp(self):
-        self.test_data = "Test Data" * 50000
+        self.test_data = b"Test Data" * 50000
         self.uploadable = MutableData(self.test_data)
 
 
     def test_datahandle_read(self):
         chunk_size = 10
-        for i in xrange(0, len(self.test_data), chunk_size):
+        for i in range(0, len(self.test_data), chunk_size):
             data = self.uploadable.read(chunk_size)
-            data = "".join(data)
+            data = b"".join(data)
             start = i
             end = i + chunk_size
             self.failUnlessEqual(data, self.test_data[start:end])
@@ -28,7 +40,7 @@ class DataHandle(unittest.TestCase):
         # disturbing the location of the seek pointer.
         chunk_size = 100
         data = self.uploadable.read(chunk_size)
-        self.failUnlessEqual("".join(data), self.test_data[:chunk_size])
+        self.failUnlessEqual(b"".join(data), self.test_data[:chunk_size])
 
         # Now get the size.
         size = self.uploadable.get_size()
@@ -38,4 +50,4 @@ class DataHandle(unittest.TestCase):
         more_data = self.uploadable.read(chunk_size)
         start = chunk_size
         end = chunk_size * 2
-        self.failUnlessEqual("".join(more_data), self.test_data[start:end])
+        self.failUnlessEqual(b"".join(more_data), self.test_data[start:end])
diff --git a/src/allmydata/test/mutable/test_different_encoding.py b/src/allmydata/test/mutable/test_different_encoding.py
index dad96f875..a5165532c 100644
--- a/src/allmydata/test/mutable/test_different_encoding.py
+++ b/src/allmydata/test/mutable/test_different_encoding.py
@@ -1,3 +1,15 @@
+"""
+Ported to Python 3.
+"""
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
+from future.utils import PY2
+if PY2:
+    from future.builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, list, object, range, str, max, min  # noqa: F401
+
 from twisted.trial import unittest
 from .util import FakeStorage, make_nodemaker
 
@@ -10,7 +22,7 @@ class DifferentEncoding(unittest.TestCase):
         # create a file with 3-of-20, then modify it with a client configured
         # to do 3-of-10. #1510 tracks a failure here
         self.nodemaker.default_encoding_parameters["n"] = 20
-        d = self.nodemaker.create_mutable_file("old contents")
+        d = self.nodemaker.create_mutable_file(b"old contents")
         def _created(n):
             filecap = n.get_cap().to_string()
             del n # we want a new object, not the cached one
@@ -19,6 +31,6 @@ class DifferentEncoding(unittest.TestCase):
             return n2
         d.addCallback(_created)
         def modifier(old_contents, servermap, first_time):
-            return "new contents"
+            return b"new contents"
         d.addCallback(lambda n: n.modify(modifier))
         return d
diff --git a/src/allmydata/test/mutable/test_filehandle.py b/src/allmydata/test/mutable/test_filehandle.py
index 547ecac41..8db02f3fd 100644
--- a/src/allmydata/test/mutable/test_filehandle.py
+++ b/src/allmydata/test/mutable/test_filehandle.py
@@ -1,21 +1,33 @@
+"""
+Ported to Python 3.
+"""
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
+from future.utils import PY2
+if PY2:
+    from future.builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, list, object, range, str, max, min  # noqa: F401
+
 import os
-from six.moves import cStringIO as StringIO
+from io import BytesIO
 from twisted.trial import unittest
 from allmydata.mutable.publish import MutableFileHandle
 
 class FileHandle(unittest.TestCase):
     def setUp(self):
-        self.test_data = "Test Data" * 50000
-        self.sio = StringIO(self.test_data)
+        self.test_data = b"Test Data" * 50000
+        self.sio = BytesIO(self.test_data)
         self.uploadable = MutableFileHandle(self.sio)
 
 
     def test_filehandle_read(self):
         self.basedir = "mutable/FileHandle/test_filehandle_read"
         chunk_size = 10
-        for i in xrange(0, len(self.test_data), chunk_size):
+        for i in range(0, len(self.test_data), chunk_size):
             data = self.uploadable.read(chunk_size)
-            data = "".join(data)
+            data = b"".join(data)
             start = i
             end = i + chunk_size
             self.failUnlessEqual(data, self.test_data[start:end])
@@ -33,7 +45,7 @@ class FileHandle(unittest.TestCase):
         # disturbing the location of the seek pointer.
         chunk_size = 100
         data = self.uploadable.read(chunk_size)
-        self.failUnlessEqual("".join(data), self.test_data[:chunk_size])
+        self.failUnlessEqual(b"".join(data), self.test_data[:chunk_size])
 
         # Now get the size.
         size = self.uploadable.get_size()
@@ -43,26 +55,26 @@ class FileHandle(unittest.TestCase):
         more_data = self.uploadable.read(chunk_size)
         start = chunk_size
         end = chunk_size * 2
-        self.failUnlessEqual("".join(more_data), self.test_data[start:end])
+        self.failUnlessEqual(b"".join(more_data), self.test_data[start:end])
 
 
     def test_filehandle_file(self):
         # Make sure that the MutableFileHandle works on a file as well
-        # as a StringIO object, since in some cases it will be asked to
+        # as a BytesIO object, since in some cases it will be asked to
         # deal with files.
         self.basedir = self.mktemp()
         # necessary? What am I doing wrong here?
         os.mkdir(self.basedir)
         f_path = os.path.join(self.basedir, "test_file")
-        f = open(f_path, "w")
+        f = open(f_path, "wb")
         f.write(self.test_data)
         f.close()
-        f = open(f_path, "r")
+        f = open(f_path, "rb")
 
         uploadable = MutableFileHandle(f)
 
         data = uploadable.read(len(self.test_data))
-        self.failUnlessEqual("".join(data), self.test_data)
+        self.failUnlessEqual(b"".join(data), self.test_data)
         size = uploadable.get_size()
         self.failUnlessEqual(size, len(self.test_data))
 
diff --git a/src/allmydata/test/mutable/util.py b/src/allmydata/test/mutable/util.py
index a664c1e08..30a75aead 100644
--- a/src/allmydata/test/mutable/util.py
+++ b/src/allmydata/test/mutable/util.py
@@ -1,4 +1,6 @@
-from six.moves import cStringIO as StringIO
+from past.builtins import long
+
+from io import BytesIO
 import attr
 from twisted.internet import defer, reactor
 from foolscap.api import eventually, fireEventually
@@ -75,8 +77,8 @@ class FakeStorage(object):
         if peerid not in self._peers:
             self._peers[peerid] = {}
         shares = self._peers[peerid]
-        f = StringIO()
-        f.write(shares.get(shnum, ""))
+        f = BytesIO()
+        f.write(shares.get(shnum, b""))
         f.seek(offset)
         f.write(data)
         shares[shnum] = f.getvalue()
@@ -129,7 +131,7 @@ class FakeStorageServer(object):
         readv = {}
         for shnum, (testv, writev, new_length) in tw_vectors.items():
             for (offset, length, op, specimen) in testv:
-                assert op in ("le", "eq", "ge")
+                assert op in (b"le", b"eq", b"ge")
             # TODO: this isn't right, the read is controlled by read_vector,
             # not by testv
             readv[shnum] = [ specimen
@@ -222,10 +224,10 @@ def make_peer(s, i):
 
     :rtype: ``Peer``
     """
-    peerid = base32.b2a(tagged_hash("peerid", "%d" % i)[:20])
+    peerid = base32.b2a(tagged_hash(b"peerid", b"%d" % i)[:20])
     fss = FakeStorageServer(peerid, s)
     ann = {
-        "anonymous-storage-FURL": "pb://%s@nowhere/fake" % (peerid,),
+        "anonymous-storage-FURL": b"pb://%s@nowhere/fake" % (peerid,),
         "permutation-seed-base32": peerid,
     }
     return Peer(peerid=peerid, storage_server=fss, announcement=ann)
@@ -297,7 +299,7 @@ def make_nodemaker_with_storage_broker(storage_broker, keysize):
 
     :param StorageFarmBroker peers: The storage broker to use.
     """
-    sh = client.SecretHolder("lease secret", "convergence secret")
+    sh = client.SecretHolder(b"lease secret", b"convergence secret")
     keygen = client.KeyGenerator()
     if keysize:
         keygen.set_default_keysize(keysize)
diff --git a/src/allmydata/test/test_uri.py b/src/allmydata/test/test_uri.py
index 3e21c1674..748a0f6ef 100644
--- a/src/allmydata/test/test_uri.py
+++ b/src/allmydata/test/test_uri.py
@@ -11,7 +11,7 @@ from __future__ import unicode_literals
 
 from future.utils import PY2
 if PY2:
-    from future.builtins import filter, map, zip, ascii, chr, dict, hex, input, next, oct, open, pow, round, super, bytes, int, list, object, range, str, max, min  # noqa: F401
+    from future.builtins import filter, map, zip, ascii, chr, dict, hex, input, next, oct, open, pow, round, super, bytes, list, object, range, str, max, min  # noqa: F401
 
 import os
 from twisted.trial import unittest
diff --git a/src/allmydata/test/test_util.py b/src/allmydata/test/test_util.py
index 14fc020b9..4e1496141 100644
--- a/src/allmydata/test/test_util.py
+++ b/src/allmydata/test/test_util.py
@@ -9,6 +9,7 @@ from __future__ import unicode_literals
 
 from future.utils import PY2
 if PY2:
+    # open is not here because we want to use native strings on Py2
     from future.builtins import filter, map, zip, ascii, chr, hex, input, next, oct, pow, round, super, bytes, dict, list, object, range, str, max, min  # noqa: F401
 import six
 import os, time, sys
diff --git a/src/allmydata/util/_python3.py b/src/allmydata/util/_python3.py
index a3a0cc647..da4b86eef 100644
--- a/src/allmydata/util/_python3.py
+++ b/src/allmydata/util/_python3.py
@@ -90,6 +90,9 @@ PORTED_MODULES = [
 ]
 
 PORTED_TEST_MODULES = [
+    "allmydata.test.mutable.test_datahandle",
+    "allmydata.test.mutable.test_different_encoding",
+    "allmydata.test.mutable.test_filehandle",
     "allmydata.test.test_abbreviate",
     "allmydata.test.test_base32",
     "allmydata.test.test_base62",