The auditory ossicles and tympanic bulla of the Archaeoceti show little intergeneric morphological variation, and are similar to the ossicles of extant cetaceans in most respects. The attachment of the tympanic membrane is separated from its origin indicating that the Archaeoceti possessed an elongated, conical tympanic conus unique to cetaceans. The sigmoid process is an expansion of the anterior limb of the ancestral tympanic bone and is entirely ectotympanic in origin. Expansion of the sigmoid process is partially responsible for the elongation of the tympanic conus by spatially separating the manubrium of the malleus out of the plane of attachment of the tympanic membrane. Elongation is further augmented by the reduction of the manubrium of the malleus, and by the evolutionary rotation of the malleus-incus system about its physiologic axis of rotation. Evolutionary rotation, reduction of the manubrium, and an expanded sigmoid process all contributed to a system of angular amplification that is still present in extant cetaceans. The specific gravity of the ossicles is greater than that of any extant cetacean for which it is known. This could have resulted from the systemic increase in bone density experienced by archaeocetes in their adaptation to the marine environment. These adaptations appear in the earliest cetacean, Pakicetus inachus, and are well developed by the Late Eocene in the Basilosauridae. Such adaptations could have laid the foundation for the development of underwater echolocation by enabling the middle ear to transmit high-frequency sound. © 1990 Taylor & Francis Group, LLC.