Rutland Water is a large (1260 ha) eutrophic pumped- storage reservoir in the lowland east midlands of England, impounded in 1975. Artificial mixing during the summer is continuous but weak, with a polymictic regime controlled by wind mixing. The crustacean zooplankton was studied during 1984 and 1985. Daphnia hyalina forma lacustrls Sars was the dominant limnetic cladoceran, with Daphnia pulex (De Geer), Bosmlna longlrostrls (O.F. Muller) and Leptodora klndtl (Focke) also present. Six limnetic copepods were present: Eudlaptomus gracilis (Sars), Eurytemora velox (Li 11jeborg), Cyclops abyssorum Sars, Cyclops vlclnus Uljanin, Acanthocyclops robustus (Sars) and Mesocyclops leuckartl (Claus). Cyclops vlclnus entered diapause as copepodite IV in May, emerging during November-March.' Reproduction was more synchronous than expected from the period of emergence and could not be explained in terms of temperature-dependent development times alone. Food availability may have limited reproduction until the spring diatom peak. The mean species richness of copepods, at 5*2, was unusually high. Species composition had changed since impoundment and this richness might represent a transition from founder species to successful immigrants. Possible mechanisms of coexistence for the calanoid species and for Cyclops species were discussed: exclusion may not occur. Features which distinguish copepodite larval instars are well known. Determination of the calanoid species was possible in all instars but the cyclopoid species were less distinct. Cyclopoids in the fifth copepodite stage showed many adult characters but earlier instars of different species did not differ qualitatively with respect to the features which were examined. The vertical distribution of Crustacea was similar to that typical of stratifying lakes, with highest densities of all groups in the upper part of the column. Agreement was found with the buoyancy/current model of large-scale horizontal patchiness. It was suggested that the effects on horizontal patchiness of *'conveyor-belt" currents should be greater in the mixed column due to a more complete coincidence of zooplankton abundance maxima and the leeward wind-driven flow.