diff --git a/src/gpb_compile.erl b/src/gpb_compile.erl
index 7cefe3ab..9217a21a 100644
--- a/src/gpb_compile.erl
+++ b/src/gpb_compile.erl
@@ -1965,7 +1965,7 @@ format_enum_decoders(Defs, #anres{used_types=UsedTypes}) ->
        [repeat_clauses('<EnumValue>',
                        [[replace_term('<EnumValue>', EnumValue),
                          replace_term('<EnumSym>', EnumSym)]
-                        || {EnumSym, EnumValue} <- EnumDef])])
+                        || {EnumSym, EnumValue} <- unalias_enum(EnumDef)])])
      || {{enum, EnumName}, EnumDef} <- Defs,
         smember({enum,EnumName}, UsedTypes)].
 
@@ -3311,7 +3311,7 @@ format_enum_value_symbol_converters(EnumDefs) when EnumDefs /= [] ->
          [repeat_clauses('<Value>',
                          [[replace_term('<Value>', EnumValue),
                            replace_term('<Sym>', EnumSym)]
-                          || {EnumSym, EnumValue} <- EnumDef])]),
+                          || {EnumSym, EnumValue} <- unalias_enum(EnumDef)])]),
        "\n",
        gpb_codegen:format_fn(
          mk_fn(enum_value_by_symbol_, EnumName),
@@ -4960,6 +4960,15 @@ map_kvalues(KVars) ->
      end
      || {Key, Expr} <- KVars].
 
+%% The "option allow_alias = true;" inside an enum X { ... }
+%% says it is ok to have multiple symbols that map to the same numeric value.
+%% Appeared in protobuf 2.5.0.
+unalias_enum([{_Sym,Value}=Enum | Rest]) ->
+    [Enum | unalias_enum([E || {_,V}=E <- Rest, V /= Value])];
+unalias_enum([{option,_Name,_Value} | Rest]) ->
+    unalias_enum(Rest);
+unalias_enum([]) ->
+    [].
 
 var_f_n(N) -> var_n("F", N).
 var_b_n(N) -> var_n("B", N).
diff --git a/src/gpb_parse.yrl b/src/gpb_parse.yrl
index 3e6b7499..09c92d91 100644
--- a/src/gpb_parse.yrl
+++ b/src/gpb_parse.yrl
@@ -101,6 +101,7 @@ enum_def -> enum identifier '{' enum_fields '}':
                                         {{enum,identifier_name('$2')},'$4'}.
 
 enum_fields -> enum_field enum_fields:  ['$1' | '$2'].
+enum_fields -> option_def enum_fields:  ['$1' | '$2'].
 enum_fields -> ';' enum_fields:         '$2'.
 enum_fields -> '$empty':                [].
 
@@ -713,7 +714,7 @@ reformat_names(Defs) ->
     lists:map(fun({{msg,Name}, Fields}) ->
                       {{msg,reformat_name(Name)}, reformat_fields(Fields)};
                  ({{enum,Name}, ENs}) ->
-                      {{enum,reformat_name(Name)}, ENs};
+                      {{enum,reformat_name(Name)}, reformat_enum_opt_names(ENs)};
                  ({{extensions,Name}, Exts}) ->
                       {{extensions,reformat_name(Name)}, Exts};
                  ({{extend,Name}, Fields}) ->
@@ -738,6 +739,16 @@ reformat_fields(Fields) ->
       end,
       Fields).
 
+%% `Defs' is expected to be parsed.
+reformat_enum_opt_names(Def) ->
+    [case Item of
+         {option, Name, Value} ->
+             {option, reformat_name(Name), Value};
+         Other ->
+             Other
+     end
+     || Item <- Def].
+
 reformat_name(Name) ->
     list_to_atom(string:join([atom_to_list(P) || P <- Name,
                                                  P /= '.'],
diff --git a/test/gpb_parse_tests.erl b/test/gpb_parse_tests.erl
index c56edf59..b8f91f08 100644
--- a/test/gpb_parse_tests.erl
+++ b/test/gpb_parse_tests.erl
@@ -181,6 +181,15 @@ parses_import_test() ->
         parse_lines(["package p1.p2;",
                      "import \"a/b/c.proto\";"]).
 
+parses_enum_option_test() ->
+    {ok, Elems} = parse_lines(["enum e1 {",
+                               "  option allow_alias = true;",
+                               "  ee1 = 1;",
+                               "  ee2 = 1;",
+                               "}"]),
+    [{{enum,e1}, [{option, allow_alias, true}, {ee1,1},{ee2,1}]}] =
+        do_process_sort_defs(Elems).
+
 generates_correct_absolute_names_test() ->
     {ok, Elems} = parse_lines(["message m1 {"
                                "  message m2 { required uint32 x = 1; }",
diff --git a/test/gpb_tests.erl b/test/gpb_tests.erl
index 588515cb..255caf2e 100644
--- a/test/gpb_tests.erl
+++ b/test/gpb_tests.erl
@@ -173,6 +173,17 @@ decode_msg_with_negative_enum_value_test() ->
                     {{enum,e}, [{v1, 100},
                                 {v2, -2}]}]).
 
+decode_msg_with_enum_aliases_test() ->
+    #m1{a = v1} =
+        decode_msg(<<8,100>>,
+                   m1,
+                   [{{msg,m1}, [#?gpb_field{name=a, fnum=1, rnum=#m1.a,
+                                            type={enum,e}, occurrence=required,
+                                            opts=[]}]},
+                    {{enum,e}, [{option, allow_alias, true},
+                                {v1, 100},
+                                {v2, 100}]}]).
+
 decode_msg_with_bool_field_test() ->
     #m1{a = true} =
         decode_msg(<<8,1>>,
@@ -462,6 +473,16 @@ encode_msg_with_negative_enum_value_test() ->
         <<254,255,255,255,255,255,255,255,255,1>> -> ok  %% encoded as 64 bits
     end.
 
+encode_msg_with_enum_aliases_test() ->
+    Defs = [{{msg,m1}, [#?gpb_field{name=a, fnum=1, rnum=#m1.a,
+                                            type={enum,e}, occurrence=required,
+                                            opts=[]}]},
+                    {{enum,e}, [{option, allow_alias, true},
+                                {v1, 100},
+                                {v2, 100}]}],
+    <<8,100>> = encode_msg(#m1{a = v1}, Defs),
+    <<8,100>> = encode_msg(#m1{a = v2}, Defs).
+
 encode_msg_with_bool_field_test() ->
     <<8,1>> =
         encode_msg(#m1{a = true},