diff --git a/src/c/file.d b/src/c/file.d
index 745187a72..422a930b3 100644
--- a/src/c/file.d
+++ b/src/c/file.d
@@ -3103,6 +3103,41 @@ set_stream_elt_type(cl_object stream, cl_fixnum byte_size, int flags,
 	stream->stream.byte_size = byte_size;
 }
 
+cl_object
+si_stream_external_format_set(cl_object stream, cl_object format)
+{
+#ifdef ECL_CLOS_STREAMS
+        if (ECL_INSTANCEP(stream)) {
+                FEerror("Cannot change external format of stream ~A", 1, stream);
+        }
+#endif
+        switch (stream->stream.mode) {
+        case smm_input:
+        case smm_input_file:
+        case smm_output:
+        case smm_output_file:
+        case smm_io:
+        case smm_io_file:
+#ifdef ECL_WSOCK
+        case smm_input_wsock:
+        case smm_output_wsock:
+        case smm_io_wsock:
+#endif
+                {
+                        cl_object elt_type = ecl_stream_element_type(stream);
+                        if (elt_type != @'character' && elt_type != @'base-char')
+                                FEerror("Cannot change external format"
+                                        "of binary stream ~A", 1, stream);
+                        set_stream_elt_type(stream, stream->stream.byte_size,
+                                            stream->stream.flags, format);
+                }
+                break;
+        default:
+                FEerror("Cannot change external format of stream ~A", 1, stream);
+        }
+        @(return)
+}
+
 cl_object
 ecl_make_file_stream_from_fd(cl_object fname, int fd, enum ecl_smmode smm,
 			     cl_fixnum byte_size, int flags, cl_object external_format)
diff --git a/src/c/symbols_list.h b/src/c/symbols_list.h
index 07c9053b0..b2b428951 100644
--- a/src/c/symbols_list.h
+++ b/src/c/symbols_list.h
@@ -1794,5 +1794,7 @@ cl_symbols[] = {
 {SYS_ "+ECL-SYNTAX-PROGV-LIST+", SI_ORDINARY, NULL, -1, OBJNULL},
 {SYS_ "WITH-ECL-IO-SYNTAX", SI_ORDINARY, NULL, -1, OBJNULL},
 
+{SYS_ "STREAM-EXTERNAL-FORMAT-SET", CL_ORDINARY, si_stream_external_format_set, 2, OBJNULL},
+
 /* Tag for end of list */
 {NULL, CL_ORDINARY, NULL, -1, OBJNULL}};
diff --git a/src/c/symbols_list2.h b/src/c/symbols_list2.h
index 0aaf719b4..b076fa8c8 100644
--- a/src/c/symbols_list2.h
+++ b/src/c/symbols_list2.h
@@ -1794,5 +1794,7 @@ cl_symbols[] = {
 {SYS_ "+ECL-SYNTAX-PROGV-LIST+",NULL},
 {SYS_ "WITH-ECL-IO-SYNTAX",NULL},
 
+{SYS_ "STREAM-EXTERNAL-FORMAT-SET","si_stream_external_format_set"},
+
 /* Tag for end of list */
 {NULL,NULL}};
diff --git a/src/h/external.h b/src/h/external.h
index fab4eff8c..f971d0128 100644
--- a/src/h/external.h
+++ b/src/h/external.h
@@ -625,6 +625,7 @@ extern ECL_API cl_object si_do_read_sequence(cl_object string, cl_object stream,
 extern ECL_API cl_object si_file_column(cl_object strm);
 extern ECL_API cl_object cl_interactive_stream_p(cl_object strm);
 extern ECL_API cl_object si_set_buffering_mode(cl_object strm, cl_object mode);
+extern ECL_API cl_object si_stream_external_format_set(cl_object strm, cl_object format);
 
 extern ECL_API bool ecl_input_stream_p(cl_object strm);
 extern ECL_API bool ecl_output_stream_p(cl_object strm);