The occunence of temperature acclimation was investigated in a range of caddis species. A closed bottle method was used for the majority of the respiration experiments, a flow-through respirometer being designed for the remaining experiments. Temperature acclimation was demonstrated in eight out of twelve species studied. Increased ability to compensate was associated with increased ecological distribution. The hydropsychids were shown to have a greater ability to acclimate than polycentropodids with similar distributions. No relationship was apparent between the interspecific differences in the respiration rate and the distribution of the species. For some species maintenance at a warmer temperature for 4-5 weeks caused a decrease in the undulatory activity of the larvae when compared, at a constant temperature, with larvae maintained at a cooler temperature. For eight of the ten species for which both sets of data were obtained the metabolic and undulatory data were compatible. A relationship between undulatory activity and distribution was suggested. For two cased caddis species a positive correlation was demonstrated between the number of gills and the body weight. Maintenance at the warmer of two temperatures over a period including a moult caused an increase in the number of gills on larvae and pupae. Intraspecific differences were found in the number of gills on larvae from different field sites, the number increasing with increased stream temperature. No relationship was demonstrated between the oxygen consumption of the larvae and the number of gills. No evidence was obtained for a difference in metabolism following maintenance of larvae at fluctuating temperatures of differing amplitudes.Decreased undulatory activity was demonstrated in larvae of Hydropsyche contuberna7is maintained under conditions of greater temperature fluctuations. Field acclimatisation was demonstrated in two species, Sericostoma personatum and Potamophy7ax cingu7atus, larvae from the warmer sites having a lower respiration rate than larvae of the same species from cooler sites, when both were measured at the same temperature.