The muskellunge Esox masquinongy is an important sport fish in Wisconsin and elsewhere, but more information about its diet is needed to better understand its role in aquatic systems and its effects on other fish. Stomach contents were examined for 1,092 muskellunge (226–1,180 mm total length, TL) captured in the littoral zone from 34 Wisconsin water bodies from Jul 1991 to Oct 1994. Food occurred in 34.3% (N = 375) of the stomachs, with most (74%) containing a single item. Overall, the proportion of muskellunge with food differed significantly among seasons, with the greatest proportion occurring in fall (69.0%), followed by summer (53.5%) and then spring (25.4%). Prey items consisted of 547 fish, representing 12 families and 31 species, along with 35 nonfish items; fish composed 98% of the diet. Relative importance values of diet items varied by taxa, season, and water body, but the main food items eaten by muskellunge in each season were yellow perch Perca flavescens and white sucker Catostomous commersoni. Black basses Micropterus spp., northern pike Esox lucius, walleye Stizostedion vitreum, cyprinids, and other taxa were less common in the diet. Prey fish ranged in size from 6% to 47% of muskellunge total length and prey length increased significantly as muskellunge size increased. Yet the size of prey in proportion to muskellunge size remained the same for all sizes of muskellunge. The results of this study indicate that, if readily available, yellow perch and catostomids will compose a large proportion of the muskellunge diet. Additional studies assessing muskellunge diet among lakes having different prey community types and assessing diet in deeper offshore areas of lakes are needed to better understand the role that muskellunge play in aquatic communities.