The development of the chironomid fauna of Cow Green reservoir in marginal sublittoral and profundal areas is described for the first few years of the reservoir's life. A total of 71 chironomid taxa in 38 genera were identified from the reservoir and its immediate environs. The number of taxa taken directly from the reservoir was 44 of which only 15 were standing‐water species. Between 1970 and 1977 there was an increase in the proportion of standing‐water species from 30 to 62.5%. Orthocladiinae, particularlyPsectrocladiusandOrthocladiusdominated marginal zones (< 1 m). Sublittoral areas (3–6 m) were characterised initially by Orthocladiinae and Tanytarsini but by the third year a more diverse community developed which included relatively high proportions of Chironomini and Tanypodinae. The clearest pattern of faunal change was observed in the profundal (15–18 m) where primary colonizers, consisting mainly of Orthocladiinae, gave way to an intermediate community dominated numerically by Tanytarsini. In the fifth year the profundal fauna was dominated by Chironomini withSergentia coracinaas the most abundant representative. The affinities of the Cow Green chironomid fauna are discussed and data are presented comparing dominant taxa in Cow Green with those observed in 18 studies in Britain and Scandinavia.