Gambel oak (Quercus gambelii) is a valuable tree species for wildlife. In Arizona, USA, it occurs at higher elevations in association with ponderosa pine (Pinus ponderosa). Southwestern myotis (Myotis auriculus) is a bat species that has been anecdotally documented as selecting Gambel oak for maternity roost sites. During summers of 1999 and 2000, we conducted a radiotelemetry study to determine whether lactating, female southwestern myotis selected Gambel oak as maternity roosts within the ponderosa pine–Gambel oak forest in northern Arizona. We located 34 maternity roosts for southwestern myotis in Gambel oak trees (14 females) and 5 roosts in ponderosa pine snags (1 female). Maternity roosts were located in live Gambel oak trees >26-cm diameter at root collar (drc) that contained cavities. We used Akaike's Information Criterion (AIC) to evaluate 4 a priori hypotheses about southwestern Myotis maternity roost selection in northern Arizona. Our top model contained parameters for roost tree height and density of potential roost trees. Roost trees were taller than randomly selected Gambel oak trees. Forest patches immediately surrounding roost trees contained a higher density of large oak trees (drc >26 cm) than around randomly selected trees. We recommend the protection of large Gambel oak trees and encourage recruitment of large Gambel oak trees from smaller size classes to ensure that roosts remain available for southwestern myotis.