ABSTRACTA survey of the variation in the eutherian auditory bulla and a consideration of its ontogeny and its possible mode of origin suggests that the basisphenoid bulla of insectivores, the petrosal bulla of primates, and the widely distributed ectotympanic bulla did not represent the condition of the most primitive eutherians. There is also evidence to refute the widely accepted premise that eutherians primitively had a cartilaginous bulla as adults. Instead, this condition is regarded as a secondary specialization derived from a bony entotympanic bulla. The entotympanic bulla was undoubtedly an early, but probably not the earliest, condition in the Eutheria. The contention that the most primitive eutherians lacked a bony or cartilaginous covering of the middle ear cavity ventrally between the petrosal and the nearly horizontal tympanic ring is supported by a number of lines of evidence. The cited existence of an entotympanic in marsupials is regarded as an independent acquisition, parallel with that in eutherians. Evidence is insufficient, in most cases, to determine whether the types of bullae in which an entotympanic is absent evolved through a stage where the latter was present, or arosede novofrom the postulated primitive eutherian condition.