diff --git a/piclib/built-in.scm b/piclib/built-in.scm
index 2ceec2e7..2dcaade4 100644
--- a/piclib/built-in.scm
+++ b/piclib/built-in.scm
@@ -457,38 +457,7 @@
 (define (list->string list)
   (apply string list))
 
-(define (string-copy! to at from . opts)
-  (let ((start (if (pair? opts) (car opts) 0))
-	(end (if (>= (length opts) 2)
-		 (cadr opts)
-		 (string-length from))))
-    (do ((i at (+ i 1))
-	 (j start (+ j 1)))
-	((= j end))
-      (string-set! to i (string-ref from j)))))
-
-(define (string-copy v . opts)
-  (let ((start (if (pair? opts) (car opts) 0))
-	(end (if (>= (length opts) 2)
-		 (cadr opts)
-		 (string-length v))))
-    (let ((res (make-string (string-length v))))
-      (string-copy! res 0 v start end)
-      res)))
-
-(define (string-fill! v fill . opts)
-  (let ((start (if (pair? opts) (car opts) 0))
-	(end (if (>= (length opts) 2)
-		 (cadr opts)
-		 (string-length v))))
-    (do ((i start (+ i 1)))
-	((= i end)
-	 #f)
-      (string-set! v i fill))))
-
-(export string string->list list->string
-        string-copy! string-copy
-        string-append string-fill!)
+(export string string->list list->string)
 
 ;;; 6.8. Vector
 
diff --git a/src/string.c b/src/string.c
index 4d5ea02f..0d9a639f 100644
--- a/src/string.c
+++ b/src/string.c
@@ -122,6 +122,52 @@ DEFINE_STRING_CMP(gt, >)
 DEFINE_STRING_CMP(le, <=)
 DEFINE_STRING_CMP(ge, >=)
 
+static pic_value
+pic_str_string_copy(pic_state *pic)
+{
+  size_t len, start, end, i;
+  char *str;
+  int n;
+  struct pic_string *copy;
+
+  n = pic_get_args(pic, "s|ii", &str, &len, &start, &end);
+
+  switch (n) {
+  case 1:
+    start = 0;
+  case 2:
+    end = len;
+  }
+
+  copy = pic_str_new(pic, NULL, end - start);
+  for (i = 0; i < end - start; ++i) {
+    copy->str[i] = str[start + i];
+  }
+  return pic_obj_value(copy);
+}
+
+static pic_value
+pic_str_string_copy_ip(pic_state *pic)
+{
+  size_t to_len, from_len, at, start, end;
+  char *to_str, *from_str;
+  int n;
+
+  n = pic_get_args(pic, "sis|ii", &to_str, &to_len, &at, &from_str, &from_len, &start, &end);
+
+  switch (n) {
+  case 3:
+    start = 0;
+  case 4:
+    end = from_len;
+  }
+
+  while (start < end) {
+    to_str[at++] = from_str[start++];
+  }
+  return pic_none_value();
+}
+
 static pic_value
 pic_str_string_append(pic_state *pic)
 {
@@ -145,6 +191,28 @@ pic_str_string_append(pic_state *pic)
   return pic_obj_value(pic_str_new(pic, buf, len));
 }
 
+static pic_value
+pic_str_string_fill_ip(pic_state *pic)
+{
+  size_t len, start, end;
+  char *str, c;
+  int n;
+
+  n = pic_get_args(pic, "sc|ii", &str, &len, &c, &start, &end);
+
+  switch (n) {
+  case 1:
+    start = 0;
+  case 2:
+    end = len;
+  }
+
+  while (start < end) {
+    str[start++] = c;
+  }
+  return pic_none_value();
+}
+
 void
 pic_init_str(pic_state *pic)
 {
@@ -153,10 +221,15 @@ pic_init_str(pic_state *pic)
   pic_defun(pic, "string-length", pic_str_string_length);
   pic_defun(pic, "string-ref", pic_str_string_ref);
   pic_defun(pic, "string-set!", pic_str_string_set);
+
   pic_defun(pic, "string=?", pic_str_string_eq);
   pic_defun(pic, "string<?", pic_str_string_lt);
   pic_defun(pic, "string>?", pic_str_string_gt);
   pic_defun(pic, "string<=?", pic_str_string_le);
   pic_defun(pic, "string>=?", pic_str_string_ge);
+
+  pic_defun(pic, "string-copy", pic_str_string_copy);
+  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);
 }