As the use of reverse osmosis (RO) for sea water desalination is rapidly expanding in Australia, the detection of biofouling on spiral wound RO membranes is becoming increasingly important for limiting this phenomenon and thus making substantial energy savings. Bacterial Regrowth Potential (BRP) and Flow Cytometry (FCM) were assessed as methods for detecting biofouling rapidly and without need for sacrificing any elements for autopsy. The study was undertaken at the Penneshaw Desalination Plant in South Australia from May 2008 to January 2009. Whilst BRP or FCM alone made interpretation of results difficult, the combination of BRP and FCM analysis provided a potential tool to evaluate membrane biofouling. Biofouling of the RO membrane was present but the extent varied due to feed seawater quality and water temperature. The dynamics of biofilms are complex as adhesion and sloughing of bacteria occur simultaneously. The combination of BRP and FCM techniques revealed sloughing or adhesion of bacteria to the membrane surface. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.