diff --git a/lib/chibi/string.sld b/lib/chibi/string.sld
index 4c13ab28..dc717165 100644
--- a/lib/chibi/string.sld
+++ b/lib/chibi/string.sld
@@ -61,7 +61,17 @@
       (define (string-cursor-prev s i) (- i 1))
       (define (substring-cursor s start . o)
         (substring s start (if (pair? o) (car o) (string-length s))))
-      (define (string-concatenate ls) (apply string-append ls))
+      (define (string-concatenate orig-ls . o)
+        (let ((sep (if (pair? o) (car o) ""))
+              (out (open-output-string)))
+          (let lp ((ls orig-ls))
+            (cond
+             ((pair? ls)
+              (if (and sep (not (eq? ls orig-ls)))
+                  (write-string sep out))
+              (write-string (car ls) out)
+              (lp (cdr ls)))))
+          (get-output-string out)))
       (define string-size string-length)
       (define (call-with-output-string proc)
         (let ((out (open-output-string)))