We used cafeteria-style feeding trials with 8 types of mast in various combinations to examine differences in resource selection among 5 syntopic species of granivorous forest rodents in west-central Indiana. Patterns of resource selection corresponded to differences in phylogeny and body size of granivores, with greatest similarities among closely related species of similar body size. Breadth of resource use varied inversely with body size in our trials. Resource selection by Sciurus carolinensis, S. niger, and Tamiasciurus hudsonicus was correlated positively with caloric and lipid content of mast and its size, whereas selection by Glaucomys volans was correlated negatively with the percentage of protective tissue associated with seeds. Laboratory trials indicated that T. hudsonicus and G. volans discriminate among mast of comparable physical and chemical composition on the basis of size, with a preference for larger seeds. Contrary to our expectations, T. hudsonicus, a recent immigrant from the boreal forest, did not exhibit a preference for seeds of red pine (Pinus resinosa) relative to mast of hardwood species. G. volans and Peromyscus leucopus tended to consume perishable white oak acorns immediately and to cache a substantial portion of less perishable seeds of nonpreferred species. Our results demonstrate the potential for considerable overlap in resource use among members of this guild. Guild members also seem to be affected differentially by physical and chemical properties of mast, and patterns of resource selection reflect contrasting foraging constraints under which these animals operate.