Peromyscus leucopus and Ochrotomys nuttalli, two small mammal species of similar life histories and body mass, exhibit a high degree of sociality and niche overlap. We investigated differences in diet preference (ingested energy, protein content and preference ranking) based on reported diets for each species in nature to better explain coexistence. Five food resources were provided to 20 adult individuals (10 male, 10 female) of each species for 10 consecutive days. Individuals were contained in separate mesocosm tanks. Dietary choices were acorns of Quercus nigra and Q. alba, seeds of Ligistrum sinense and Rhus typhina, and fruits of Cornus florida. Golden mice preferred fruits of C. florida and acorns of Q. nigra, and to a lesser extent acorns of Q. alba, whereas white-footed mice consumed acorns of both Q. nigra and Q. alba and fruits of C. florida in equal amounts. White-footed mice consumed significantly more energy per day than golden mice (2.38 and 1.48 Kcal • g live wt⁻¹, respectively). Thus, P. leucopus, considered a habitat generalist species, exhibited a wider dietary breadth and greater caloric intake than O. nuttalli, considered a habitat specialist species.