Diatoms are well known contributors to the biofouling of seawater reverse osmosis (SWRO) membranes however there have been no studies on the isolation of specific diatoms from native sea water and their subsequent impact in a laboratory scale cross-flow situation. SWRO feed tank water was analyzed for microorganisms, nutrients and transparent exopolymer substances (TEPs). Subsequently, the diatom genus, Pseudo-nitzschia sp., was isolated and used in a laboratory scale cross-flow system as a model phytoplankton species for TEP production. The biofilm produced after 7 days was then characterized, as well as the nutrients and TEP. The average quantity of TEP found on the biofouled membrane was 28.09 ± 3.40 μg XG eq L -1 , which is lower than that of the feed water over the duration of the cross-flow experiment (3.5-5.1 × 10 3 μg XG eq L -1 ). Proteins, lipids and polysaccharide biofilm compounds were present on the RO membranes, as well as diatoms and bacteria. The total abundance of pico-phytoplankton increased over the experimental period with proliferation of Prochlorococcus being the major contributor to this increase.