Colonization of glass substrata by populations of three or four bacterial species over periods of four weeks or more was investigated using recirculating, model laboratory systems. Numbers of coryneform, Aeromonas hydrophila, Pseudomonas fluorescens, and Xanthomonas maltophilia on surfaces and in the liquid phase were monitored to determine whether any species inhibited or facilitated the colonization by another organism. The system was 800 ml of culture in a peptone/yeast/extract/artificial lakewater medium, pulse-fed every three days, and recirculated 12 ml/min. Species numbers were determined by viable counts on differential media, and biofilm coverage was evaluated by computer-enhanced microscopy. The coryneform quickly colonized the surfaces, and there was little change in suspended or attached numbers over the experimental period. Colonization by A. hydrophila increased in the presence of P. fluorescens. The results indicated that the ability of bacteria to colonize surfaces is to a large extent related to their ability to colonize the liquid phase.