As part of a study of the distribution and possible biological consequences of insecticides in Lake Michigan, the reproductive success of Herring Gulls at Bellow Island, Grand Traverse Bay, and Pismire Island of the Beaver Islands was estimated in 1965. At Bellow Island the hatch of eggs was low (30%) owing to egg cracking and apparent abnormalities of behavior in the care of the eggs by adult birds. Gulls nesting at Bellow Island were exceptionally aggressive. The fledging rate at Bellow was estimated at 0.42 chick per pair of adults. At Pismire Island, where nesting appeared to be normal throughout the year, an estimated 80% of the eggs hatched and the fledging rate was 1.22 chicks per pair of adults. Insecticide residues in adult gulls averaged 22% greater at Bellow Island. Residues in eggs from both areas approximated 120 ppm and probably cannot be blamed for the reduced hatchability of eggs at Bellow Island. Reports on other aspects of this problem are being prepared by Hickey and Keith.