The breeding success of Larus argentatus and L. delawarensis was monitored at South Manitou Island in Lake Michigan from 1975 through 1979. During this period red fox predation was severe during four of the five breeding seasons. As a result, heavy losses of eggs, young and adult gulls occurred in both colonies. Each year, except 1977, reproductive success was near zero. The two species have responded differently to such heavy losses. So far neither species has abandoned the traditional sites used on the island. The Ring-bills, however, have exhibited considerable reduction in colony size. Observations to date suggest that Herring Gulls express a greater tendency toward site tenacity than do Ring-billed Gulls. Data describing the impact of foxes on the two species and the subsequent changes in colony characteristics are discussed.