diff --git a/README.md b/README.md
index 46e9e19a..29e88c42 100644
--- a/README.md
+++ b/README.md
@@ -141,7 +141,7 @@ At the REPL start-up time, some usuful built-in libraries listed below will be a
 | 6.4 Pairs and lists | yes | `list?` is safe for using against circular list. |
 | 6.5 Symbols | yes | |
 | 6.6 Characters | yes | |
-| 6.7 Strings | yes | `substring` is not provided |
+| 6.7 Strings | yes | |
 | 6.8 Vectors | yes | |
 | 6.9 Bytevectors | yes | |
 | 6.10  Control features | yes | |
diff --git a/src/string.c b/src/string.c
index a9d7891e..edaf1edc 100644
--- a/src/string.c
+++ b/src/string.c
@@ -416,4 +416,5 @@ pic_init_str(pic_state *pic)
   pic_defun(pic, "string-copy!", pic_str_string_copy_ip);
   pic_defun(pic, "string-append", pic_str_string_append);
   pic_defun(pic, "string-fill!", pic_str_string_fill_ip);
+  pic_defun(pic, "substring", pic_str_string_copy);
 }