As a signatory of the Conservation Assessment and Conservation Strategy for Swift Fox, the state of New Mexico is required to manage swift foxes (Vulpes velox) with special diligence. However, basic ecological data for the swift fox in New Mexico are lacking. I examined swift fox demography, home range size, dispersal, den site selection, and diet in northeastern New Mexico. Juveniles comprised the most numerous age class. Body size measurements were generally as large or larger, and weights were smaller, than those reported from more northerly areas. Traumatic injury, presumably by coyotes (Canis latrans), was the primary cause of death. Annual survival rates for adults averaged 0.53. Of 36 swift foxes captured during the study, 4 remained alive on the study area, 21 died, and 11 left the study area by the end of 32 months of field work. The annual 95% MCP home range size estimate was 1,494.5 ha. Diet was dominated by invertebrates and mammals. For den sites, swift foxes preferred the vicinity of roads, areas with greater road density, low slope, hilltops, and sandy loam and clay soils.