Lipolytic bacteria were isolated from two stations on Brewery Creek, an arm of the Ottawa River, during the winter of 1971-72. Total counts were approximately sevenfold higher at the more polluted downstream station, whereas lipolytic counts were about 100-fold higher. At this station, significantly more lipolytic bacteria grew on plates incubated at 20 C than at 4 C, suggesting that the population was comprised of both mesophiles and psychrophiles. However, at the upstream station, approximately the same number were obtained at both temperatures. A total of 434 isolates, mainly from the downstream station, were tentatively classified. The major groups were Pseudomonas, Acinetobacter-Moraxella, and Aeromonas. Though the total number of lipolytic bacteria was fairly constant throughout the winter, the relative abundance of the acinetobacters dropped from approximately 90% in November to less than 10% in March, and then increased. The aeromonads and pseudomonads showed the opposite trend. Most of the bacteria, though isolated at 4 C, also grew at 30 C. Lipolysis, however, was generally strongest at 20 C or below.