Breeding population, diet, and reproductive success of Double-crested Cormorants (Phalacrocorax auritus) on Lake Winnipegosis, Manitoba, were studied from 15 May to 6 September, 1987. We recorded 35,181 nests on 37 colonies, a dramatic increase over the 1979 estimate of 9053 nests on 17 colonies. Eleven colonies, located primarily at the north end of the lake, accounted for 70% of the total number of breeding pairs. Evidence of human disturbance was found at 15 colonies. These disturbed colonies showed a mean productivity of approximately 0.4 .+-. 05 (SD; n = 13 colonies) mobile chicks/nest compared with 1.8 .+-. 0.9 (n = 19 colonies) mobile chicks/nest at undisturbed colonies. Evidence of Bald Eagle (Haliaeetus leucocephalus) predation of hatch-year birds was found at 4 colonies. We collected 6169 prey samples representing 16 fish and one crayfish species. Yellow Perch (Perca flavescens) were found most frequently but White Sucker (Catostomus commersoni) made up nearly half the prey biomass. Walleye (Stizostedion vitreum) and Sauger (S. canadense), accounted for only 0.1 and 0.2% of the prey biomass, respectively. We suggest that increases in populations of forage fish such as suckers, due primarily to overfishing of Walleye and pike, may have contributed to the cormorant population increase.