3. An account is given of sound conductivity experiments which were carried out on the auditory structures in a very fresh, dead specimen of Tursiops. 4. The probable function of the external auditory meatus is discussed in relationship to the arrangement of the accessory air sinuses of the middle ear. 5. The so-called ”auditory scanning” behaviour in odontocete cetaceans is commented upon in the light of preliminary experimental evidence of a vocal sound diffraction pattern.