diff --git a/README.md b/README.md
index b838fc1..a6c54f9 100644
--- a/README.md
+++ b/README.md
@@ -44,6 +44,8 @@ options.  Some of the more important are:
 
 -k, --keep-core-mods  
   Prevent swapping of core mods.  Only change the values of existing mods.
+  Specifying this option twice prevents downgrading mods from the original
+  values.
 
 -i, --income  
   Optimize for runestone income instead of damage.  This is much slower so it
diff --git a/spire.cpp b/spire.cpp
index b01a6e7..fde6227 100644
--- a/spire.cpp
+++ b/spire.cpp
@@ -78,6 +78,7 @@ Spire::Spire(int argc, char **argv):
 	budget(0),
 	core_budget(0),
 	core_mutate(Core::ALL_MUTATIONS),
+	no_core_downgrade(false),
 	income(false),
 	towers(false),
 	score_func(damage_score)
@@ -99,7 +100,7 @@ Spire::Spire(int argc, char **argv):
 	bool exact = false;
 	std::string budget_str;
 	std::string core_budget_str;
-	bool keep_core_mods = false;
+	unsigned keep_core_mods = 0;
 	std::string tower_type;
 	unsigned towers_seen = 0;
 
@@ -219,7 +220,10 @@ Spire::Spire(int argc, char **argv):
 	}
 
 	if(keep_core_mods)
+	{
 		core_mutate = Core::VALUES_ONLY;
+		no_core_downgrade = (keep_core_mods>=2);
+	}
 
 	if(!n_pools_seen && heterogeneous)
 		n_pools = 21;
@@ -1078,7 +1082,16 @@ void Spire::Worker::main()
 				Core core = mutated.get_core();
 				core.mutate(spire.core_mutate, 1+random()%5, random);
 				core.update();
-				if(core.cost<=spire.core_budget)
+
+				bool ok = (core.cost<=spire.core_budget);
+
+				if(ok && spire.no_core_downgrade)
+				{
+					for(unsigned j=0; (ok && j<Core::N_MODS); ++j)
+						ok = (core.get_mod(j)>=mutated.get_core().get_mod(j));
+				}
+
+				if(ok)
 					mutated.set_core(core);
 			}
 
diff --git a/spire.h b/spire.h
index fce56bb..b621dca 100644
--- a/spire.h
+++ b/spire.h
@@ -72,6 +72,7 @@ class Spire
 	Number budget;
 	Number core_budget;
 	Core::MutateMode core_mutate;
+	bool no_core_downgrade;
 	bool income;
 	bool towers;
 	Pool::ScoreFunc *score_func;