I studied microgeographic and size-related variation in the diet composition of the Northern Watersnake Nerodia sipedon sipedon in the Beaver Archipelago of Lake Michigan. Snakes from Lake Michigan shoreline sites were piscivorous whereas snakes from interior sites on Beaver Island ate mostly amphibians. Sculpin (Cottus sp.) were the dominant prey of all size classes, but a gradual size-related shift in diet may be occurring in archipelago snakes. Large snakes tended to eat larger prey than small snakes and to drop the smallest prey from their diets, but overlap in prey size occurred among all size classes of snakes. The diet composition of snakes from different sites is likely a reflection of local differences in prey abundance, as found in several previous studies. The overlap in prey size and prey type among snake size classes contradicts the results of some previous studies of Nerodia sp. and merits further investigation.