diff --git a/src/V3DfgPeephole.cpp b/src/V3DfgPeephole.cpp
index 7c0fc7e25..8181dcfab 100644
--- a/src/V3DfgPeephole.cpp
+++ b/src/V3DfgPeephole.cpp
@@ -138,6 +138,7 @@ template <> const DfgDataType& resultDType<DfgXor>        (const DfgVertex* lhsp
 
 // Unary constant folding
 template<typename Vertex> void foldOp(V3Number& out, const V3Number& src);
+template <> void foldOp<DfgCountOnes>  (V3Number& out, const V3Number& src) { out.opCountOnes(src); }
 template <> void foldOp<DfgExtend>     (V3Number& out, const V3Number& src) { out.opAssign(src); }
 template <> void foldOp<DfgExtendS>    (V3Number& out, const V3Number& src) { out.opExtendS(src, src.width()); }
 template <> void foldOp<DfgLogNot>     (V3Number& out, const V3Number& src) { out.opLogNot(src); }
@@ -1195,6 +1196,10 @@ class V3DfgPeephole final : public DfgVisitor {
     //  DfgVertexUnary
     //=========================================================================
 
+    void visit(DfgCountOnes* const vtxp) override {
+        if (foldUnary(vtxp)) return;
+    }
+
     void visit(DfgExtend* const vtxp) override {
         if (foldUnary(vtxp)) return;
 
diff --git a/test_regress/t/t_dfg_peephole.v b/test_regress/t/t_dfg_peephole.v
index 9e5c64da4..3c9ce7ae8 100644
--- a/test_regress/t/t_dfg_peephole.v
+++ b/test_regress/t/t_dfg_peephole.v
@@ -62,6 +62,7 @@ module t (
   //verilator lint_on WIDTH
   `signal(FOLD_UNARY_Extend, tmp_FOLD_UNARY_Extend);
   `signal(FOLD_UNARY_ExtendS, tmp_FOLD_UNARY_ExtendS);
+  `signal(FOLD_UNARY_CountOnes, $countones(const_a));
   `signal(FOLD_UNARY_OneHot,    $onehot(const_a));
   `signal(FOLD_UNARY_OneHot0,   $onehot0(const_a));
   `signal(FOLD_UNARY_OneHot_A,  $onehot(const_a[0]));