SUMMARY.1. Water samples were collected over two years from the Yorkshire Ouse and Yorkshire Derwent and the following were measured: (i) concentration of directly‐counted bacteria (free‐living and particle‐bound), (ii) concentration of colony‐forming units, (iii) bacterial heterotrophic activity (turnover rate for glucose assimilation), (iv) specific activity (turnover rate per bacterium), (v) a range of environmental variables.2. The abundance and activity of suspended bacteria showed similar seasonal periodicities in both rivers.3. Free‐living bacteria were usually more numerous than particle‐bound bacteria; low concentration of free‐living bacteria and maxima of particle‐bound bacteria usually occurred in winter.4. Concentration of colony‐forming units varied irregularly, but lowest levels were found in summer.5. Turnover rate and turnover rate per bacterium showed distinct summer maxima.6. Multiple‐regression analysis was used to relate bacterial variables to subsets (chosen by factor analysis) of environmental variables; up to 89% of variation in bacterial variables was related to the combined effects of variation by variables in