The successional changes in a new. lowland reservoir in Leicestershire, England, were investigated, with particular refere~ce to larvae of the Chironomidae. During the first four years major changes in water chemistry were the result of the different sources of water used to fill the reservoir and the release of nutrients from the inundated terrestrial vegetation. Phytoplankton populations showed erratic changes in species composition and standing crop during the first year of filling. Seasonal fluctuations, similar to those observed at other eutrophic reservoirs, occurred in subsequent years. Chironomid species composition and temporal and spatial variation in the larval populations were investigated. Orth6cladiinae larvae were particularly abundant in mats of algae in shallow water in the second year after filling commenced. This was probably the result of a stable water level, warm weather conditions and high nutrient concentrations. In the third year Chironomus p lumosus and Polypedi tum nubeculosum larvae were numerically do~nant. Populations of these spec~es declined the following year and Tanytarsus species predominated. Temporal changes of the fauna were lnfluenced by climate and the filling regime of the reservoir. Chironamid larvae did not indicate any marked differences in water quali ty be tween the two arms of the reservoir. Populations were generally found to be contagious ly dis tributed. Chironomids in rainbow and brown trout diets were investigated during the first two fishing seasons. The species composition of larvae and pupae in the diets suggested different feeding zones for the two trout species. The behaviour of chironomid larvae and not their numerical abundance determines their occurrence in the diets. The results from this study are discussed in relation to management of the reservoir. Due to the importance of chironomids in trout diet it may be beneficial to base stocking policy on a knowledge of the seasonal population fluctuations of the chironomids.