diff --git a/source/ConfigSetup.h b/source/ConfigSetup.h
index b821cb4f..a5677f37 100644
--- a/source/ConfigSetup.h
+++ b/source/ConfigSetup.h
@@ -48,6 +48,7 @@ EMP_BUILD_CONFIG(SymConfigBase,
     VALUE(INT_VAL_MUT_RATE, double, -1, "The overall mutation rate of the interaction value trait in symbionts, -1 makes it same as mutation rate as determined by tranmission mode"),
     VALUE(HORIZ_MUTATION_SIZE, double, -1, "Standard deviation of the distribution to mutate by for horizontal transmission, if -1 MUTATION_SIZE used"),
     VALUE(HORIZ_MUTATION_RATE, double, -1, "Value 0 to 1 of probability of mutation for horizontal transmission, if -1 MUTATION_RATE used"),
+    VALUE(SYM_WITHIN_LIFETIME_MUTATION_RATE, double, 0, "Value 0 to 1 of probability of mutation for symbiont every time step"),
 
     GROUP(ECTOSYMBIOSIS, "Settings for ectosymbiosis and free-living symbionts"),
     VALUE(FREE_LIVING_SYMS, bool, 0, "Are symbionts able to live outside of the host?"),
diff --git a/source/default_mode/Symbiont.h b/source/default_mode/Symbiont.h
index 6b50360f..3e6db9f7 100644
--- a/source/default_mode/Symbiont.h
+++ b/source/default_mode/Symbiont.h
@@ -516,7 +516,12 @@ class Symbiont: public Organism {
     HorizontalTransmission(location);
     //Age the organism
     GrowOlder();
-    //Check if the organism should move and do it
+    if (my_config->SYM_WITHIN_LIFETIME_MUTATION_RATE()) {
+      if (random->P(my_config->SYM_WITHIN_LIFETIME_MUTATION_RATE())) {
+        Mutate();
+      }
+    }
+    //Check if the organism should move and do it 
     if (my_host.IsNull() && my_config->FREE_LIVING_SYMS() && !dead) {
       //if the symbiont should move, and hasn't been killed
       my_world->MoveFreeSym(location);