The present paper is concerned with the comparative morphology of the archeocete and odontocete skull. Among the archeocetes, the recently described lower Eocene Pakicetus inachus obviously represents an early stage of adaptation to aquatic life. The morphology of the incomplete cranial remains, however, gives no evidence that Pakicetus was an amphibious intermediate stage. The evolution of advanced archeocetes and odontocetes is characterized by the successive acquirement of new morphological devices related to the emission and perception of ultrasound under water. The formation of a sonar system in odontocetes obviously not only helped to compensate for the loss of the peripheral olfactory system but moreover was a substantial factor in the evolution of the exceptional dolphin brain.