Diptera from Grenada Reservoir (40 km2) were collected monthly from February, 1973 through August, 1975. Fourteen genera and 13 species were identified. Aquatic stages ofChaoborus punctipemis(Say),Coelotanypus tricolor(Loew), andChironomus attenuatus(Walker) constituted over 90% of all dipterans collected. Diptera were distributed throughout the reservoir and constituted the single most important component of the macrobenthos. Certain species were ubiquitous while others were restricted by depth or substrate. Changes in water temperature, dissolved oxygen, and water level controlled emergence, repopulation and larval migrations. Populations showed mid-winter highs and stunner lows with some species having as many as four generations per year.