diff --git a/src/main/startup.c b/src/main/startup.c
index 1d2a6755b5a..59c87af32ab 100644
--- a/src/main/startup.c
+++ b/src/main/startup.c
@@ -34,6 +34,11 @@
 #include <unistd.h>
 #endif
 
+#ifdef __APPLE__
+# include <sys/types.h>
+# include <sys/sysctl.h>
+#endif
+
 /* These are used in ../gnuwin32/system.c, ../unix/sys-std.c */
 SA_TYPE SaveAction = SA_SAVEASK;
 SA_TYPE	RestoreAction = SA_RESTORE;
@@ -251,6 +256,17 @@ void R_SizeFromEnv(Rstart Rp)
 	R_size_t MinMaxVSize = 17179869184; /* 16 Gb */
 	Rp->max_vsize = sysmem > MinMaxVSize ? sysmem : MinMaxVSize;
     }
+#elif defined(__APPLE__) && (SIZEOF_SIZE_T > 4)
+    else {
+	R_size_t sysmem = 0;
+	R_size_t len = sizeof(sysmem);
+	if (!sysctlbyname("hw.memsize", &sysmem, &len, NULL, 0)
+	    && len == sizeof(sysmem)) {
+
+	    R_size_t MinMaxVSize = 17179869184; /* 16 Gb */
+	    Rp->max_vsize = sysmem > MinMaxVSize ? sysmem : MinMaxVSize;
+	}
+    }
 #endif
     if((p = getenv("R_VSIZE"))) {
 	value = R_Decode2Long(p, &ierr);