In a survey of Esthwaite Water and Windermere (North Basin) lakes an attempt was made to determine the factors which control the bacterial populations during the period of stratification. The variables estimated included pH, temperature, dissolved oxygen concentration, turbidity, particulate matter, phosphate and nitrate, rainfall, windspeed, chlorophyll a, viable bacteria, specific exoenzyme-producing bacteria, and the levels of certain enzymes in the water samples. Correlation coefficients for the variables were calculated to determine, in particular, whether a relationship between the algal and bacterial populations could be detected. In the more nutrient-rich lake (Esthwaite) the major factors which appeared to control the bacterial population were temperature, dissolved oxygen concentration and pH. In Windermere a positive correlation was noted not only with temperature and pH but also with particulate matter and rainfall. No correlation between viable bacteria and chlorophyll a levels was noted but the bacterial population and enzyme activity occasionally increased with or soon after phytoplankton maxima, which suggested some response.