Understanding mechanisms that influence the grouping tendencies of large herbivores is necessary to predict the influence of environmental and human factors on threatened populations. Locations of 53 adult female pronghorn (Antilocapra americana) in Yellowstone National Park during June 1999–April 2005 indicated that mean and typical group sizes and the variation in group size decreased during fawning when females secluded themselves, but became larger and more dynamic during fawn rearing and the rut and winter. Mixed-effects models indicated a strong effect of time of year on mean group sizes, with some evidence that predators negatively affected group sizes during winter. Within-animal variability (0.64) was substantially higher than between-animal variability (0.02). Pronghorn density, snow water equivalent, and predation apparently influenced variations in group size. Multiple regressions indicated effects of pronghorn density and snow water equivalent on typical group size, the size of the group in which the average animal finds itself. Overall, there was fluidity in group cohesion, with female associations changing within and among days. The behavioral plasticity of pronghorn with respect to grouping and social cohesion might confer resilience to changes in environmental conditions, but often makes it difficult to predict the consequences of conservation actions to control disease, protect or restore key habitat, regulate harvests, and limit adverse effects of development and recreation.