In social ungulates particular individuals or cohorts, such as adult females, can lead or initiate foraging movements. We use muskoxen (Ovibos moschatus) as a model system to test hypotheses regarding the sex and age class of leaders and the potential costs of leadership in 3 different behavioral contexts: activity initiation, foraging-bout movements, and spontaneous group movements. We conducted research on approximately 160 muskoxen at Cape Krusenstern National Monument adjacent to the coast of the Chuckchi Sea in northwestern Alaska. We observed free-ranging, mixed-sex groups of muskoxen from the ground during 5 June–22 September 2002. Adult females emerged as leaders in all 3 contexts, and other group members were more likely to follow adult females than adult males during initiations of activity. Half of successful initiations by adult males included aggressive behavior toward females. Males took a more active role during rut (the mating season) by provoking females to initiate group activity and by actively manipulating spontaneous movements led by females through herding and blocking activities. Leaders incurred no obvious costs in terms of lost foraging time. Costs and benefits of leadership can be subtle and difficult to measure in the field and can include foraging and reproductive trade-offs and increased risk of predation.