1. The annual cycle of Cyclops strenuus abyssorum Sars in some lakes of the English Lake District is described. Esthwaite Water, Rydal Water and Grasmere, all enriched by sewage, and Buttermere, a relatively unproductive lake, were sampled most frequently. 2. There is only one effective generation in the year in each of these lakes. Individuals of this generation hatch from eggs laid in the spring. In the deepest of the four lakes (Buttermere), these individuals reach the adult stage in early winter, pass the winter in the planktonic zone and start laying eggs early in the following year. In the other three lakes, most individuals of the spring generation reach the fifth copepodid stage by mid-summer and then spend the next eight months in the profundal zone. They leave this zone in February or March to return to the planktonic zone, become adult and start breeding. In July a few individuals may mature in the planktonic zone and give use to a second small generation in the autumn. 3. A correlation between the time in mid-summer when stage V copepodids appear in the profundal zone of the three shallower lakes and amounts of dissolved oxygen in this zone is shown. Although a correlation of this kind is plausible, presence of stage V copepodids in a deep, well oxygenated lake (Thirlmere) suggests that a causal connection of this kind is too simple an explanation, and perhaps only coincidental. 4. Rates of development of the different instars were found experimentally at constant temperatures and used to estimate production. Production, biomass and fecundity were all higher in Esthwaite Water than in the other three lakes.