diff --git a/include/functions b/include/functions
index c1df0fbf..e3a09c0c 100644
--- a/include/functions
+++ b/include/functions
@@ -1284,7 +1284,8 @@
 
         if [ -z "${search}" ]; then ExitFatal "Missing process to search for when using IsRunning function"; fi
         RUNNING=0
-        if [ ! -z "${PGREPBINARY}" ]; then
+        # AIX does not fully support pgrep options, so using ps instead
+        if [ ! -z "${PGREPBINARY}" -a ! "${OS}" = "AIX" ]; then
             FIND=$(${PGREPBINARY} ${pgrep_options} "${search}" | ${TRBINARY} '\n' ' ')
         else
             if [ -z "${PSOPTIONS}" ]; then