Abstract: The red fox population inhabiting California's Cascade and Sierra Nevada mountains (Vulpes vulpes necator) is listed as a State Threatened species, but its management has been hindered by a lack of basic ecological information. I conducted a comprehensive study of the red foxes in the Lassen Peak region to quantify their local distribution, resource utilization, activity patterns, niche overlap with likely competitors and genetic affinity with other red fox populations. The population was restricted to the region's highest elevations, occurring >1300 m and primarily within the western half of Lassen Volcanic National Park. Red fox detections at camera traps in summer were positively correlated with elevation, highway density and the detection of coyotes, and were negatively correlated with shrub and herbaceous cover; in winter, detections were positively correlated with elevation, highway density and mature closed-canopy forest cover. Their diet was predominantly mammals, especially rodents and mule deer (Odocoileus hemionus), supplemented by birds, insects and manzanita (Arctostaphylos nevadensis ) berries as seasonally available. Lagomorphs were virtually absent from the fox diet. Collared red foxes (n = 5) had large seasonal home ranges (95% MCP; mean = 2,564 ha in summer and 3,255 ha in winter). On average, summer locations were 479 m higher than winter locations. Their summer range is likely unsuitable in winter due to deep soft snow and the lack of lagomorphs, a critical winter food for many other red fox populations, and these factors may limit the Lassen red fox population. Marten (Martes americana) used the same habitat as the foxes but preyed upon smaller rodents and were more diurnal. Coyotes (Canis latrans) were nocturnal like the foxes but were generally at lower elevations and ate larger prey. The Lassen foxes all had the same mtDNA haplotype, which also was the most common haplotype among historic V. v. necator specimens and was rare in the exotic fox populations from California's lowlands. Ecological and genetic evidence indicates that the Lassen red foxes are the native, threatened V. v. necator, not exotic foxes dispersed from the lowlands. Additional research is necessary to locate additional mountain red fox populations in California and to identify the factors preventing their dispersal to the lowlands and vice versa.