Regurgitated food samples were collected from Ring-billed Gulls at 2 breeding colonies on Lake Huron and 1 on Lake Michigan during May through July of 1964 and 1971. Fish comprised 76% of the total diet volume, insects 22%, and earthworms 1%. By frequency of occurrence, fish appeared in 61% of the individual samples, insects in 64%, and earthworms in 5%. Frequency and volumetric data are provided for each major taxon. Changes in diet were noted as the breeding season progressed. In May, June, and July, fish constituted 56%, 72% and 84%, respectively, while for the same 3 months insects accounted for 2%, 28%, and 16% of the volume. Earthworms contributed 26% of the total May volume but declined to 1% in June and nothing in July. Food types in samples were indicative of aquatic, aerial, and terrestrial foraging. Food habits are discussed in relation to gull population dynamics on the Great Lakes. Dietary differences were noted between gulls nesting at mainland and offshore sites. Fish accounted for 69% of the volume at Rogers City and 88% at Ile aux Galets; insects contributed 30% of the total volume at Rogers City and 11% at Ile aux Galets. There was a greater diversity of food items in the Ile aux Galets samples as well as a larger percentage of food of aquatic origin. The periods of peak abundance of the 3 fish species in the samples roughly coincided with the respective spawning seasons.