Archaeocete whales of the subfamily Dorudontinae (Cetacea, Basilosauridae) show strong evidence of having replaced upper and lower 1st deciduous premolars with permanent teeth late in their dental eruption sequence. This situation is rare in modern mammals, many of which have no teeth in the 1st premolar position or which retain a deciduous tooth in the 1st premolar position that is not replaced. Evidence for replacement of deciduous 1st premolars in dorudontine archaeocetes comes from both juvenile and adult specimens of Dorudon atrox from Egypt and Zygorhiza kochii from North America. Replacement of deciduous 1st premolars by permanent teeth may have been precipitated by a general delay in skeletal maturation found in cetaceans, which allowed dental development to proceed for a longer period of time than is generally available for most mammals.