The carbon dioxide output in respiration of Pisidium casertanum, P. henslowanum and P. conventus was studied in the laboratory. A closedbottle method was used, and the amount of dissolved inorganic carbon was measured with an infrared gas analyser. The mean value of the constant b in equation R=aW b, where R is the respiration rate and W the ash-free dry weight of the animal, was ca. 0.75 in both the eurytherm P. casertanum and the cold-stenotherm P. conventus at five temperatures, but the temperature dependence of b was opposite in these species. The temperature dependence of metabolism increased with increasing size in P. casertanum and P. henslowanum but decreased in P. conventus. A 'reversed acclimatization pattern' was found, where the metabolism rate and its temperature dependence was lower in the cold-adapted stenotherm species.