SUMMARY.Five populations ofAsellus aquaticus, subjected to different combinations of temperature and organic pollution in the River Trent, were examined over a period of 18 months. The life‐cycle was probably completed in a year or less in all the populations sampled, a larger proportion of the animals completing the life‐cycle in less than a year at warmer locations. Young animals also hatched 1 or 2 months earlier at the warmer locations.At the locations subjected to most organic pollution, population densities and the size of animals of both sexes were the largest. The size of ovigerous females, however, appeared to be inversely related to temperature, the smallest occurring at the warmest location. Possible explanations for these differences are given in terms of the amount of food available and the effect of temperature on growth and sexual maturat