The formation of biofilms on reverse osmosis membranes was studied applying tap water (<5 cfu colonization forminf unit/mL), a FT 30 polyamide membrane, and a flat-channel cell (80 cm2 surface for sepn.). The expts. were carried out at 4 MPa, a cross flow of 150 L/h, and 15 Deg. The approx. rate of surface coverage was 30% after 1 day and 100% after 3 days of operation; then, the colonization came to a plateau. The permeate prodn. decreased correspondingly and reached a plateau as well. With increasing microorganism levels in water, both the surface colonization and the permeate prodn. reached a new plateau. It is assumed that every reverse osmosis system carries a biofilm which participates in the sepn. process as secondary membrane. Salt rejection data, however, did not reflect initial biofilm formation.