Leptotragulus, from the upper middle Eocene of North America, is the most plesiomorphic member of the Protoceratidae, an enigmatic group of selenodont artiodactyls. Until now Leptotragulus was known primarily from teeth, limb bones and a few skull fragments. A previously undescribed series of crania shows that Leptotragulus exhibits the typical protoceratid skull morphology, with a broad, flattened forehead, pronounced lambdoid crests enclosing a deep complexus fossa, and broad occipital condyles with a pronounced anteroventral margin. It is suggested that these features, which are suited to forms of agonistic, intraspecific behaviour such as head-butting, may indicate that the evolution of such behaviour occurred comparatively early in the history of the protoceratids. Study of the anatomy of the otic region in Leptotragulus suggests that there are a number of similarities in periotic morphology between leptotragulines and plesiomorphic ruminants such as hypertragulids and leptomerycids, which may indicate a sister group relationship between the Protoceratidae and the Ruminantia.