diff --git a/src/machi.proto b/src/machi.proto
index d4c3b82..59c9242 100644
--- a/src/machi.proto
+++ b/src/machi.proto
@@ -74,7 +74,7 @@ message Mpb_ChunkCSum {
 // epoch_id() type
 message Mpb_EpochId {
     required uint32 epoch_num = 1;
-    required Mpb_ChunkCSum epoch_csum = 2;
+    required bytes epoch_csum = 2;
 }
 
 //////////////////////////////////////////
@@ -135,12 +135,11 @@ message Mpb_WriteChunkReq {
     required string file = 1;
     required uint64 offset = 2;
     required bytes chunk = 3;
+    required Mpb_ChunkCSum csum = 4;
 }
 
 message Mpb_WriteChunkResp {
     required Mpb_GeneralStatusCode status = 1;
-    // If OK, then chunk_pos is defined.
-    optional Mpb_ChunkPos chunk_pos = 2;
 }
 
 //////////////////////////////////////////
diff --git a/src/machi_pb_high_client.erl b/src/machi_pb_high_client.erl
index 6529bca..b076959 100644
--- a/src/machi_pb_high_client.erl
+++ b/src/machi_pb_high_client.erl
@@ -38,7 +38,8 @@
          connected_p/1,
          echo/2, echo/3,
          auth/3, auth/4,
-         append_chunk/6, append_chunk/7
+         append_chunk/6, append_chunk/7,
+         write_chunk/5, write_chunk/6
         ]).
 
 %% gen_server callbacks
@@ -82,6 +83,12 @@ append_chunk(PidSpec, PlacementKey, Prefix, Chunk, CSum, ChunkExtra) ->
 append_chunk(PidSpec, PlacementKey, Prefix, Chunk, CSum, ChunkExtra, Timeout) ->
     send_sync(PidSpec, {append_chunk, PlacementKey, Prefix, Chunk, CSum, ChunkExtra}, Timeout).
 
+write_chunk(PidSpec, File, Offset, Chunk, CSum) ->
+    write_chunk(PidSpec, File, Offset, Chunk, CSum, ?DEFAULT_TIMEOUT).
+
+write_chunk(PidSpec, File, Offset, Chunk, CSum, Timeout) ->
+    send_sync(PidSpec, {write_chunk, File, Offset, Chunk, CSum}, Timeout).
+
 send_sync(PidSpec, Cmd, Timeout) ->
     gen_server:call(PidSpec, {send_sync, Cmd}, Timeout).
 
@@ -195,14 +202,7 @@ do_send_sync({append_chunk, PlacementKey, Prefix, Chunk, CSum, ChunkExtra},
         PK = if PlacementKey == <<>> -> undefined;
                 true                 -> PlacementKey
              end,
-        CSumT = case CSum of
-                    none ->
-                        #mpb_chunkcsum{type='CSUM_TAG_NONE',
-                                       csum=undefined};
-                    {client_sha, CSumBin} ->
-                        #mpb_chunkcsum{type='CSUM_TAG_CLIENT_SHA',
-                                       csum=CSumBin}
-                end,
+        CSumT = convert_csum_req(CSum),
         Req = #mpb_appendchunkreq{placement_key=PK,
                                   prefix=Prefix,
                                   chunk=Chunk,
@@ -224,20 +224,59 @@ do_send_sync({append_chunk, PlacementKey, Prefix, Chunk, CSum, ChunkExtra},
     catch X:Y ->
             Res = {bummer, {X, Y, erlang:get_stacktrace()}},
             {Res, S#state{count=Count+1}}
+    end;
+do_send_sync({write_chunk, File, Offset, Chunk, CSum},
+             #state{sock=Sock, sock_id=Index, count=Count}=S) ->
+    try
+        ReqID = <<Index:64/big, Count:64/big>>,
+        CSumT = convert_csum_req(CSum),
+        Req = #mpb_writechunkreq{file=File,
+                                 offset=Offset,
+                                 chunk=Chunk,
+                                 csum=CSumT},
+        R1a = #mpb_request{req_id=ReqID,
+                           write_chunk=Req},
+        Bin1a = machi_pb:encode_mpb_request(R1a),
+        ok = gen_tcp:send(Sock, Bin1a),
+        {ok, Bin1B} = gen_tcp:recv(Sock, 0),
+        case (catch machi_pb:decode_mpb_response(Bin1B)) of
+            #mpb_response{req_id=ReqID, write_chunk=R} when R /= undefined ->
+                Result = convert_write_chunk_resp(R),
+                {Result, S#state{count=Count+1}};
+            #mpb_response{req_id=ReqID, generic=G} when G /= undefined ->
+                #mpb_errorresp{code=Code, msg=Msg, extra=Extra} = G,
+                {{error, {Code, Msg, Extra}}, S#state{count=Count+1}}
+        end
+    catch X:Y ->
+            Res = {bummer, {X, Y, erlang:get_stacktrace()}},
+            {Res, S#state{count=Count+1}}
     end.
 
+convert_csum_req(none) ->
+    #mpb_chunkcsum{type='CSUM_TAG_NONE',
+                   csum=undefined};
+convert_csum_req({client_sha, CSumBin}) ->
+    #mpb_chunkcsum{type='CSUM_TAG_CLIENT_SHA',
+                   csum=CSumBin}.
+
 convert_append_chunk_resp(#mpb_appendchunkresp{status='OK', chunk_pos=CP}) ->
     #mpb_chunkpos{offset=Offset, chunk_size=Size, file_name=File} = CP,
     {ok, {Offset, Size, File}};
-convert_append_chunk_resp(#mpb_appendchunkresp{status='BAD_ARG'}) ->
+convert_append_chunk_resp(#mpb_appendchunkresp{status=Status}) ->
+    convert_general_status_code(Status).
+
+convert_general_status_code('BAD_ARG') ->
     {error, bad_arg};
-convert_append_chunk_resp(#mpb_appendchunkresp{status='WEDGED'}) ->
+convert_general_status_code('WEDGED') ->
     {error, wedged};
-convert_append_chunk_resp(#mpb_appendchunkresp{status='BAD_CHECKSUM'}) ->
+convert_general_status_code('BAD_CHECKSUM') ->
     {error, bad_checksum};
-convert_append_chunk_resp(#mpb_appendchunkresp{status='PARTITION'}) ->
+convert_general_status_code('PARTITION') ->
     {error, partition};
-convert_append_chunk_resp(#mpb_appendchunkresp{status='BAD_JOSS'}) ->
+convert_general_status_code('BAD_JOSS') ->
     throw({error, bad_joss_taipan_fixme}).
 
-
+convert_write_chunk_resp(#mpb_writechunkresp{status='OK'}) ->
+    ok;
+convert_write_chunk_resp(#mpb_writechunkresp{status=Status}) ->
+    convert_general_status_code(Status).
diff --git a/test/machi_pb_high_client_test.erl b/test/machi_pb_high_client_test.erl
index 73b35a5..1a3c0ed 100644
--- a/test/machi_pb_high_client_test.erl
+++ b/test/machi_pb_high_client_test.erl
@@ -69,9 +69,9 @@ smoke_test2() ->
             CSum2 = {client_sha, machi_util:checksum_chunk(Chunk2)},
             {ok, {Off2, Size2, File2}} =
                 ?C:append_chunk(Clnt, PK, Prefix, Chunk2, CSum2, 1024),
-            %% Chunk3 = ["This is a ", <<"test,">>, 32, [["Hello, world!"]]],
-            %% {ok, {Off3, Size3, File3}} =
-            %%     ?C:write_chunk(Clnt, File2, Off2+iolist_size(Chunk2), Chunk3),
+            Chunk3 = ["This is a ", <<"test,">>, 32, [["Hello, world!"]]],
+            ok = ?C:write_chunk(Clnt, File2, Off2+iolist_size(Chunk2),
+                                Chunk3, none),
 
             ok
         after