Mountain Bluebirds (Sialia currucoides) occupy an extensive breeding range in western North America (Bent 1949). They breed in a greater variety of habitats, including the Upper Sonoran, Transition, Canadian and Hudsonian life zones, than other members of the genus (Power 1966, Herlugson 1978, Pinkowski 1979a). Although Ponderosa Pine (Pinus ponderosa) savannas are often used by breeding Mountain Bluebirds (Pinkowski 1979a), there are apparently no published reports of the reproductive biology of the species in Ponderosa Pine-dominated habitats. Here I report results of a 3-year study of Mountain Bluebird reproductive biology in a Ponderosa Pine savanna in southcentral Montana.