diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5e3d4db0..88fd6c31 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,7 +4,7 @@
 
 Features
 
-- Added `read-bytevector` and `write-bytevector` IO functions from R7RS.
+- Added `read-bytevector`, `read-bytevector!`, and `write-bytevector` I/O functions from R7RS.
 
 Bug Fixes
 
diff --git a/scheme/base.sld b/scheme/base.sld
index c55bced4..52905e7b 100644
--- a/scheme/base.sld
+++ b/scheme/base.sld
@@ -735,16 +735,25 @@
                            (loop (+ n 1)))
                           (else bv))))))
         (loop 0)))
-    ;; TODO: need to finish and debug the following function:
-    (define (read-bytevector! vec . opts)
-      (letrec ((len (bytevector-length vec))
-               (port (if (> (length opts) 0) (car opts) (current-output-port)))
-               (start (if (> (length opts) 1) (cadr opts) 0))
-               (end (if (> (length opts) 2) (caddr opts) len))
-               (bv (read-bytevector (- end start) port))
-               )
-      (bytevector-copy! vec start bv)
-      (- end start))) ;; TODO: return number of bytes read
+    (define (read-bytevector! vec . o)
+      (let* ((in (if (pair? o) (car o) (current-input-port)))
+             (o (if (pair? o) (cdr o) o))
+             (start (if (pair? o) (car o) 0))
+             (end (if (and (pair? o) (pair? (cdr o)))
+                      (cadr o)
+                      (bytevector-length vec))))
+        (if (>= start end)
+            0
+            (let ((res (read-bytevector (- end start) in)))
+              (cond
+               ((eof-object? res)
+                res)
+               (else
+                (let ((len (bytevector-length res)))
+                  (do ((i 0 (+ i 1)))
+                      ((>= i len) len)
+                    (bytevector-u8-set! vec (+ i start) (bytevector-u8-ref res i))
+                    ))))))))
     (define (write-bytevector vec . opts)
       (letrec ((len (bytevector-length vec))
                (port (if (> (length opts) 0) (car opts) (current-output-port)))