Successful translocation of a listed species into an area of previous occupation requires knowledge of the habitat needs. The presence of the necessary food items is critical to the successful establishment of a new population; this information is unknown for Peromyscus polionotus niveiventris, the southeastern beach mouse, a threatened subspecies on the east coast of Florida. I used fecal and stable isotope analysis to determine the diet of this subspecies at Cape Canaveral Air Force Station, Brevard County, Florida, between the autumn of 2003 and the spring of 2005. Six trapping grids were established, three in the dune/swale and three in the coastal scrub communities. Fecal and hair samples were collected and analyzed. The diet varied in the amount of 13C consumed between habitats and in the amount of both 15N and 13C consumed among grids within a habitat. There was no significant interaction between habitat and sex in the amount of either 15N or 13C consumed, and sexes also did not differ significantly. Fecal analysis uncovered the dominance in the diet of C3 plants. My data refuted the current belief, that the southeastern beach mouse prefers beach grass seeds of C4 plants, which were consumed but not in the frequency or quantity expected. I also analyzed the diet of Peromyscus gossypinus, the cotton mouse, and Sigmodon hispidus, the hispid cotton rat, using the two techniques. Both species consumed a combination of plant and arthropod material. Their diets varied between dune/swale and coastal scrub habitats. All three species' diets were significantly different, with Peromyscus polionotus niveiventris and Peromyscus gossypinus being the most similar. Both consume a greater proportion of arthropod material compared to the hispid cotton rat. Interspecific competition between the southeastern beach mouse and the cotton mouse may occur in times of limited resources.