The food habits of river otters (Lontra canadensis) on three rivers in the Red River of the North drainage of eastern North Dakota were evaluated using an analysis of 569 scats collected between 4 Oct. 2006 and 26 Nov. 2007. Fish and crayfish were the primary prey items, occurring in 83.0% and 51.1% of scats, respectively. Other prey included insects (26.7%), birds (7.9%), amphibians (6.7%), mammals (6.0%) and freshwater mussels (0.2%). Fish of Cyprinidae (carp and minnows) were the most prominent fish in the diet, occurring in 64.7% of scats. Other relatively common fish in the diet included Ictaluridae (catfish, 17.4% frequency of occurrence), Catostomidae (suckers, 13.0%), and Centrarchidae (sunfish, 11.2%). The diet of river otters changed seasonally, including a decline in the frequency of fish in the summer diet, and a corresponding increase in the occurrence of crayfish. Consumed fish ranged from 3.5 to 71.0 cm total length, with a mean of 20.7 cm (se ± 0.5, n = 658). Fish 10.1–20.0 cm were the most frequently consumed size class (36.5% relative frequency), with the majority of other consumed fish being ≤10.0 cm (24.6%), 20.1–30.0 cm (14.1%), 30.1–40.0 cm (14.0%), or 40.1–50.0 cm (8.2%). The size of consumed fish changed seasonally, with spring having the largest mean prey size.