diff --git a/src/main/fart.cc b/src/main/fart.cc
index 3ceb2f2..7797bad 100644
--- a/src/main/fart.cc
+++ b/src/main/fart.cc
@@ -96,11 +96,16 @@ static void * render_thread(void * varg)
 static void renderThreaded(Scene & scene,
                            unsigned char * data,
                            int width,
-                           int height)
+                           int height,
+                           int num_threads)
 {
-    int num_threads = sysconf(_SC_NPROCESSORS_ONLN);
+    /* a non-positive override means auto-detect the available cores */
     if (num_threads < 1)
-        num_threads = 1;
+    {
+        num_threads = sysconf(_SC_NPROCESSORS_ONLN);
+        if (num_threads < 1)
+            num_threads = 1;
+    }
 
     render_thread_state_t state;
     state.scene = &scene;
@@ -132,6 +137,7 @@ void usage(const char * progname)
     cout << "    -d|--max-depth <max-recursion-depth>" << endl;
     cout << "    -a|--ambient-occlusion <ambient-occlusion-level>" << endl;
     cout << "    -p|--preview  (means -w400 -h300 -m1 -d8 -a0)" << endl;
+    cout << "    -j|--threads <num-threads>  (default: number of cores)" << endl;
     cout << "       --hosts <hosts-file>" << endl;
     exit(42);
 }
@@ -194,6 +200,7 @@ int main(int argc, char * argv[])
     unsigned char * data = NULL;
     const char * output_file_name = "fart.bmp";
     bool child = false;
+    int num_threads = 0;        /* 0 means auto-detect available cores */
 
     static const struct option long_options[] = {
         { "ambient-occlusion",  required_argument, NULL, 'a' },
@@ -204,6 +211,7 @@ int main(int argc, char * argv[])
         { "field-of-view",      required_argument, NULL, 'f' },
         { "max-depth",          required_argument, NULL, 'd' },
         { "preview",            no_argument,       NULL, 'p' },
+        { "threads",            required_argument, NULL, 'j' },
         { "help",               no_argument,       NULL, 256 },
         { "host",               required_argument, NULL, 257 },
         { "port",               required_argument, NULL, 258 },
@@ -212,7 +220,7 @@ int main(int argc, char * argv[])
         { NULL,                 0,                 NULL, 0   }
     };
 
-    while ((opt = getopt_long(argc, argv, "a:o:w:h:m:f:d:p",
+    while ((opt = getopt_long(argc, argv, "a:o:w:h:m:f:d:pj:",
                               long_options, &option_index)) != -1)
     {
         switch (opt)
@@ -258,6 +266,9 @@ int main(int argc, char * argv[])
                 scene_options["max-depth"] = "8";
                 client_options.push_back("--preview");
                 break;
+            case 'j':
+                num_threads = atoi(optarg);
+                break;
             case 256:
                 usage(argv[0]);
                 break;
@@ -378,8 +389,8 @@ int main(int argc, char * argv[])
     }
     else
     {
-        /* local multithreaded render using all available cores */
-        renderThreaded(scene, data, width, height);
+        /* local multithreaded render; num_threads of 0 auto-detects cores */
+        renderThreaded(scene, data, width, height, num_threads);
     }
 
     gettimeofday(&after, NULL);         /* stop timing */