The fate of organic pollutants in aquatic ecosystems depends, in part, on metabolic activities of the indigenous microflora. Knowledge is therefore needed for the growth characteristics of aquatic bacteria in low nutrient conditions typical of many aquatic environments. The authors have studied changes in bacterial populations from Lake Ontario, using continuous culture enrichments, with lactose as the sole carbon and energy source. Enrichment studies have shown that the concentrations of lactose, the type of water utilized for inflowing media, and the inoculum source affected the outcome of continuous culture enrichments from fresh-water samples. Two isolates from enrichments experiments, an Aeromonas and a Pseudomonas, were obtained using concentrations of 50 and 5 mg/liter lactose, respectively. These isolates were shown to be competitive with each other at different lactose concentrations.