Among the several hundred bones of sauropod dinosaurs recovered from the Poison Creek Quarry in the Morrison Formation of north central Wyoming is the finely preserved skull of a young diplodocid. In most aspects it appears to belong to the genus Diplodocus. However, its rostrum and lower jaw narrow anteriorly and present an unusual skull form previously unreported for this taxon. Observations of the skull suggest several possibilities: 1) deformation is responsible for its narrow snout; 2) the rostrum may reflect a sexual character and the relatively few known Diplodocus skulls which appear to have wider snouts may represent a single sex; 3) the condition is one of immaturity and would change with age; 4) the condition is indicative of a new taxon. Little is understood about the variability and ontogenetic development of the Diplodocidea. Complete preparation of this new specimen adds to our knowledge of this group.