In this study, membrane biofouling caused by bacteria that have different characteristics was evaluated using now field-flow fractionation (FIFFF). Three different bacteria which differed from size and shape (Staphylococcus epidermidis, Escherichia coli, Flavobacterium lutescens) were investigated with GM ultrafiltration (UF, rough with a low negative surface charge and relatively high hydrophobicity) and NE70 nanofiltration (NF, smooth with a high negative surface charge and relatively low hydrophobicity) membranes. The FIFFF retention time of S. epidermidis, E. coli and F. lutescens was highly influenced by the ionic strength of the solution and the surface polarity of the membranes and bacteria. The NF membrane was found to have a higher potential of biofouling than the UF membrane with the bacteria tested in this study. E. coli was the most significant biofoulant among the bacteria tested on both membrane surfaces based on FIFFF retention times compared to other bacteria. (C) 2009 Elsevier Ltd. All rights reserved