diff --git a/ChangeLog b/ChangeLog
index d832787..06fd1f8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2003-08-06  Richard Kreckel  <kreckel@ginac.de>
+
+	* src/numtheory/cl_nt_sqrtmodp.cc: #undef _R.
+	Reported by Andrew Rechnitzer <A.Rechnitzer@ms.unimelb.edu.au>.
+
 2003-08-01  Richard Kreckel  <kreckel@ginac.de>
 
 	More dependent base resolution issues
diff --git a/src/numtheory/cl_nt_sqrtmodp.cc b/src/numtheory/cl_nt_sqrtmodp.cc
index 098c406..cf2556e 100644
--- a/src/numtheory/cl_nt_sqrtmodp.cc
+++ b/src/numtheory/cl_nt_sqrtmodp.cc
@@ -16,6 +16,9 @@
 #include <cmath>
 #define floor cln_floor
 
+// MacOS X does "#define _R 0x00040000L".  Grr...
+#undef _R
+
 namespace cln {
 
 // Algorithm 1 (for very small p only):