A stirred cell was used to study initial adhesion of three sewage bacteria belonging to the genus Pseudomonas to the three reverse osmosis (RO) membranes BW30, PVD and CAB2, and the nanofiltration membrane NF45. Membranes were immersed in suspensions containing 10(8) bacteria/ml for 10 min. All three strains were capable of rapidly colonising the four membranes, but to different extents. It was found that bacteria would sometimes aggregate upon adhering to particular RO membranes. The effects of solution ionic strength and pH, and conditioning of membranes (by prior exposure to filtrates of treated and untreated sewage) on the number of adherent bacteria were investigated. Minimal bacterial attachment occurred in a very low ionic strength milieu (deionised water). Salt concentrations corresponding to waste water and to twice that concentration resulted in significantly higher but statistically similar numbers of attached microbes. Adhesion of he three isolates was not affected by pH in the range of 4-8. The number of bacteria attaching to the membranes could be increased or reduced by conditioning films of sewage origin, conditioning films could also trigger or inhibit aggregation of adherent cells. Some surface properties of the membranes (roughness, hydrophobicity) and bacterial cells (electrophoretic mobility, functional groups by affinity chromatography) were also investigated.