diff --git a/ChangeLog.markdown b/ChangeLog.markdown
index 57ecbd93..6fda46ec 100644
--- a/ChangeLog.markdown
+++ b/ChangeLog.markdown
@@ -9,6 +9,7 @@ New Features:
 Bug Fixes:
 
 - Fixed `rational?` to properly handle floating-point numbers.
+- Migrated `string-fill!` from a macro to a function. This makes it easier to redefine, for example per SRFI 13.
 
 v3.19.1
 --------
diff --git a/lib/core.scm b/lib/core.scm
index 880949ea..48fa332c 100644
--- a/lib/core.scm
+++ b/lib/core.scm
@@ -331,11 +331,17 @@
 ; End delayed evaluation section
 
 ; String Section
-(define-syntax string-fill!
-  (syntax-rules ()
-    ((_ _str _chr)
-     (set! _str
-           (make-string (string-length _str) _chr)))))
+(define (string-fill! str fill . opts)
+  (letrec* ((len (string-length str))
+            (start (if (> (length opts) 0) (car opts) 0))
+            (end (if (> (length opts) 1) (cadr opts) len))
+            (loop (lambda (i)
+                    (cond
+                     ((= i end) str)
+                     (else
+                       (string-set! str i fill)
+                       (loop (+ i 1)))))))
+    (loop start)))
 
 (define (string-concatenate l) 
     (apply string-append l))