diff --git a/src/PDF.cpp b/src/PDF.cpp
index af88cf45..9f144508 100644
--- a/src/PDF.cpp
+++ b/src/PDF.cpp
@@ -181,7 +181,7 @@ namespace StochHMM{
             std::cerr << "Poisson PMF: Incorrect parameters\n";
             exit(2);
         }
-        return (pow(lambda,(double)k)* exp(-1*lambda))/factorial(k);
+        return (pow(lambda,(double)k)* exp(-1*lambda))/tgamma((double)k+1.0);
     }