Transparent exopolymer particles (TEP) are sticky organic microgels, ubiquitous in natural waters, which have been implicated as a potentially important factor in the development of aquatic biofilm. An experimental cross-flow membrane array was used to investigate the relationship between the rate of membrane clogging and levels of TEP and other water quality variables in a lake water source. In three experimental series, feedwater TEP concentrations correlated significantly with membrane fouling rate. To check whether feedwater TEP could be a source for extracellular polymeric substances (EPS) of early biofilm, 5 experiments were run with either untreated (active bacteria) or chlorinated (inactivated bacteria) feedwater. Confocal scanning laser microscopy and image analysis of biofilm on membranes after 50 h showed similar EPS in the biofilm, irrespective of whether the bacteria were similar to 98% inactivated or fully active. This would indicate that most of the EPS appearing at early stages of biofilm on membranes originated from TEP in the feedwater rather than from metabolizing bacteria adhering to the surface. Taken together, these experiments support the premise that TEP in source waters play a significant role in the early stages of aquatic biofilm formation and are an important causative factor in membrane fouling. (C) 2011 Elsevier B.V. All rights reserved.