diff --git a/core/String.carp b/core/String.carp
index d6316502..0cdea927 100644
--- a/core/String.carp
+++ b/core/String.carp
@@ -1,5 +1,13 @@
 (system-include "carp_string.h")
 
+(system-include "ctype.h")
+(defmodule Byte
+  (hidden   tolower-)   ; helper func for String.ascii-to-lower
+  (register tolower- (Fn [Byte] Byte) "tolower")
+  (hidden   toupper-)   ; helper func for String.ascii-to-upper
+  (register toupper- (Fn [Byte] Byte) "toupper")    
+)
+
 (defmodule String
 
   (register =          (Fn [&String &String] Bool))
@@ -169,6 +177,13 @@
 
   (doc contains? "Checks whether the string `s` contains the character `c`.")
   (defn contains? [s c] (> (index-of s c) -1))
+
+  (doc  ascii-to-lower "converts each character in this string to lower case using tolower() from standard C library. Note: this will only work for ASCII characters.")
+  (defn ascii-to-lower [s]
+    (String.from-bytes &(Array.endo-map &(fn [c] (Byte.tolower- c)) (String.to-bytes s))) )
+  (doc  ascii-to-upper "converts each character in this string to upper case using toupper() from standard C library. Note: this will only work for ASCII characters.")
+  (defn ascii-to-upper [s]
+    (String.from-bytes &(Array.endo-map &(fn [c] (Byte.toupper- c)) (String.to-bytes s))) )
 )
 
 (defmodule StringCopy
@@ -501,3 +516,4 @@
         (cons (String.head s) (String.to-list (String.tail s)))))
   )
 )
+
diff --git a/test/string.carp b/test/string.carp
index 0ca339d0..1598a29c 100644
--- a/test/string.carp
+++ b/test/string.carp
@@ -323,4 +323,12 @@
                 &[\� \�]
                 &(chars &(from-bytes &[255b 255b]))
                 "check for invalid UTF-8 sequences")
+  (assert-equal test
+                "hällo wörld"
+                &(ascii-to-lower "HälLo WöRld")
+                "ascii-to-lower works for valid UTF-8" )
+  (assert-equal test
+                "HäLLO WöRLD"
+                &(ascii-to-upper "HälLo WöRld")
+                "ascii-to-upper works for valid UTF-8" )
 )