diff --git a/src/ctobssim/ctobssim.cpp b/src/ctobssim/ctobssim.cpp
index b7a6be8..e069e74 100644
--- a/src/ctobssim/ctobssim.cpp
+++ b/src/ctobssim/ctobssim.cpp
@@ -322,6 +322,9 @@ void ctobssim::save(void)
             // Save file
             obs->save(filename, clobber());
 
+            // Increment file number
+            file_num++;
+
         } // endif: observation was a CTA observation
 
     } // endfor: looped over files