diff --git a/csug/numeric.stex b/csug/numeric.stex
index 00c45d44c8..e7ba98d1eb 100644
--- a/csug/numeric.stex
+++ b/csug/numeric.stex
@@ -1378,9 +1378,9 @@ bits in a fixnum, i.e.,
 
 %----------------------------------------------------------------------------
 \entryheader
-\formdef{fxpopcount}{\categoryprocedure}{(fxpopcount \var{fixnum} \dots)}
-\formdef{fxpopcount32}{\categoryprocedure}{(fxpopcount32 \var{fixnum} \dots)}
-\formdef{fxpopcount16}{\categoryprocedure}{(fxpopcount16 \var{fixnum} \dots)}
+\formdef{fxpopcount}{\categoryprocedure}{(fxpopcount \var{fixnum})}
+\formdef{fxpopcount32}{\categoryprocedure}{(fxpopcount32 \var{fixnum})}
+\formdef{fxpopcount16}{\categoryprocedure}{(fxpopcount16 \var{fixnum})}
 \returns number of bits set in \var{fixnum}
 \listlibraries
 \endentryheader
@@ -1391,6 +1391,11 @@ bits in a fixnum, i.e.,
 \var{fixnum} is treated as a two's complement integer, regardless
 of the underlying representation.
 
+See also \scheme{fxbit-count}, which produces the same result as
+\scheme{fxpopcount} for the domain of \scheme{fxpopcount}. Because
+\scheme{fxbit-count} also handles negative arguments, however, it does
+not map as simply to certain processor instructions.
+
 
 \section{Random Number Generation\label{SECTNUMERICRANDOM}}
 
diff --git a/s/5_3.ss b/s/5_3.ss
index ae5746f768..f2f88f7eba 100644
--- a/s/5_3.ss
+++ b/s/5_3.ss
@@ -2489,23 +2489,9 @@
       (make-count-table)))
   (define $fxbit-count
     (lambda (n)
-      (if (fx= n 0)
-          0
-          (constant-case ptr-bits
-            [(64)
-             (fx+ (bytevector-u8-ref count-table (fxlogand n #xff))
-                  (bytevector-u8-ref count-table (fxlogand (fxsrl n 8) #xff))
-                  (bytevector-u8-ref count-table (fxlogand (fxsrl n 16) #xff))
-                  (bytevector-u8-ref count-table (fxlogand (fxsrl n 24) #xff))
-                  (bytevector-u8-ref count-table (fxlogand (fxsrl n 32) #xff))
-                  (bytevector-u8-ref count-table (fxlogand (fxsrl n 40) #xff))
-                  (bytevector-u8-ref count-table (fxlogand (fxsrl n 48) #xff))
-                  (bytevector-u8-ref count-table (fxlogand (fxsrl n 56) #xff)))]
-            [(32)
-             (fx+ (bytevector-u8-ref count-table (fxlogand n #xff))
-                  (bytevector-u8-ref count-table (fxlogand (fxsrl n 8) #xff))
-                  (bytevector-u8-ref count-table (fxlogand (fxsrl n 16) #xff))
-                  (bytevector-u8-ref count-table (fxlogand (fxsrl n 24) #xff)))]))))
+      (if (fx< n 0)
+          (fxnot (fxpopcount (fxnot n)))
+          (fxpopcount n))))
   (define $big-bit-count
     (lambda (n)
       (let ([end (fx+ (fx* ($bignum-length n) (constant bigit-bytes)) (constant bignum-data-disp))])