Sonoran pronghorns (Antilocapra americana sonoriensis) have been listed as endangered since 1967, yet relatively little is known about their behavior. Knowledge of behavior is essential to understanding their ecology, but these data are difficult to obtain for Sonoran pronghorns due to land-use restrictions and large, inaccessible range. However, with establishment of a semi-captive breeding facility (260 ha) for Sonoran pronghorns in southern Arizona, behavioral studies are feasible. We studied diurnal activity budgets of six adult female Sonoran pronghorns during January–December 2005 by recording behavior (i.e., foraging, bedding, standing, traveling, other) with instantaneous sampling at 2-min intervals. We observed pronghorns on 148 days during the year for a total of 49,035 observations. Pronghorns exhibited seasonal and diurnal differences in activity budgets. Behavior was constrained by temperature in spring and summer, and by availability of forage in autumn. These data are the first regarding detailed behavior of Sonoran pronghorns and are valuable to their management and conservation by providing more information on constraints faced by the subspecies.