We have investigated biofouling behavior in forward osmosis (FO) and reverse osmosis (RO) membrane systems. Analysis of these two systems was done via comparison of biofilm structure and membrane permeate water flux decline. Experiments were performed using a model wastewater solution inoculated with Pseudomonas aeruginosa in laboratory-scale FO and RO test cells. For a meaningful comparison, biofouling runs were carried out using the same thin-film composite polyamide FO membrane, identical hydrodynamic conditions, and an initial permeate water flux of 19 L m(-2) h(-1). Water flux decline after 600 mL of cumulative permeate volume (or similar to 17-18 h of fouling) was significantly lower in FO (similar to 10%) compared to RO (similar to 30%). FO biofilms grew in a loosely organized thick layer, with eminent mushroom-shaped structures, while RO biofilms grew in tightly organized mats encased in larger amounts of extracellular polymeric substances (EPS) per cell. The more compact biofilms in RO induced greater biofilm-enhanced osmotic pressure and hydraulic resistance to water flow compared to FO, which resulted in higher flux decline. We attribute the differences in biofouling behaviors in FO and RO to the different driving forces: osmotic pressure in FO and hydraulic pressure in RO. (C) 2015 Elsevier B.V. All rights reserved.