Botfly infestation was analyzed in 1319 small mammals from varied habitats and elevations in upland Virginia, 1972-1974. Adult Cuterebra fontinella were reared from bots infesting the woodmouse (Peromyscus leucopus). Of 651 woodmice, 199 (30.6%) were infested at least once. The season of bot infestation extended from May through November of all years, with peak prevalence (mean 42.2%) in August. Infestation in woodmice was general over a 2000 km2 study area during peak months. Significantly greater prevalence (P less than 0.001) was seen in forested and brushy habitat (39.9%) compared with old fields (2.6%). Higher prevalence (P less than 0.01) was found in males than in females, and in postjuveniles compared to adults. Greatest overall prevalence (39.1%) was in postjuvenile males. Most warble sites (98%) were inguinal, and observations suggested that many infested adults were non-reproductive. Mean burden (1.27) was independent of host age, sex, habitat, and month of capture. Multi-infestations occurred in 23% of the affected mice; maximum burden was six bots. Botflies were absent or negligible in sympatric deermice (P. maniculatus) and twelve other species of rodents and shrews.