diff --git a/src/allmydata/storage/http_client.py b/src/allmydata/storage/http_client.py
index 5e964bfbe..697af91b5 100644
--- a/src/allmydata/storage/http_client.py
+++ b/src/allmydata/storage/http_client.py
@@ -198,7 +198,7 @@ class StorageClientImmutables(object):
             "/v1/immutable/{}/{}".format(_encode_si(storage_index), share_number)
         )
         response = yield self._client._request(
-            "POST",
+            "PATCH",
             url,
             upload_secret=upload_secret,
             data=data,
diff --git a/src/allmydata/storage/http_server.py b/src/allmydata/storage/http_server.py
index 23f0d2f1c..28367752d 100644
--- a/src/allmydata/storage/http_server.py
+++ b/src/allmydata/storage/http_server.py
@@ -219,15 +219,35 @@ class HTTPServer(object):
     )
     def write_share_data(self, request, authorization, storage_index, share_number):
         """Write data to an in-progress immutable upload."""
-        # TODO parse the content-range header to get offset for writing
-        # TODO basic checks on validity of offset
-        # TODO basic check that body isn't infinite. require content-length? if so, needs t be in protocol spec.
+        storage_index = si_a2b(storage_index.encode("ascii"))
+        content_range = request.getHeader("content-range")
+        if content_range is None:
+            offset = 0
+        else:
+            offset = int(content_range.split()[1].split("-")[0])
+
+        # TODO basic checks on validity of start, offset, and content-range in general. also of share_number.
+        # TODO basic check that body isn't infinite. require content-length? or maybe we should require content-range (it's optional now)? if so, needs to be rflected in protocol spec.
+
         data = request.content.read()
-        # TODO write to bucket at that offset.
+        try:
+            bucket = self._uploads[storage_index].shares[share_number]
+        except (KeyError, IndexError):
+            # TODO return 404
+            raise
 
-        # TODO check if it conflicts with existing data (probably underlying code already handles that) if so, CONFLICT.
+        finished = bucket.write(offset, data)
 
-        # TODO if it finished writing altogether, 201 CREATED. Otherwise 200 OK.
+        # TODO if raises ConflictingWriteError, return HTTP CONFLICT code.
+
+        if finished:
+            request.setResponseCode(http.CREATED)
+        else:
+            request.setResponseCode(http.OK)
+
+        # TODO spec says we should return missing ranges. but client doesn't
+        # actually use them? So is it actually useful?
+        return b""
 
     @_authorized_route(
         _app,
diff --git a/src/allmydata/storage/immutable.py b/src/allmydata/storage/immutable.py
index da9aa473f..5878e254a 100644
--- a/src/allmydata/storage/immutable.py
+++ b/src/allmydata/storage/immutable.py
@@ -13,7 +13,7 @@ if PY2:
 
 import os, stat, struct, time
 
-from collections_extended import RangeMap
+from collections_extended import RangeMap, MappedRange
 
 from foolscap.api import Referenceable
 
@@ -375,7 +375,10 @@ class BucketWriter(object):
     def allocated_size(self):
         return self._max_size
 
-    def write(self, offset, data):
+    def write(self, offset, data):  # type: (int, bytes) -> bool
+        """
+        Write data at given offset, return whether the upload is complete.
+        """
         # Delay the timeout, since we received data:
         self._timeout.reset(30 * 60)
         start = self._clock.seconds()
@@ -399,6 +402,10 @@ class BucketWriter(object):
         self.ss.add_latency("write", self._clock.seconds() - start)
         self.ss.count("write")
 
+        # Return whether the whole thing has been written.
+        # TODO needs property test
+        return self._already_written.ranges() == [MappedRange(0, self._max_size, True)]
+
     def close(self):
         precondition(not self.closed)
         self._timeout.cancel()