ABSTRACT Hereditary absence of otoliths occurs in some mice homozygous for the mutant pallid. The development of the ears of pallid mice, with and without otoliths, was compared with that of normal mice. In ears lacking otoliths all parts of the developing otoliths could be detected, except the calcium salts. Abnormality was first detected at days’ gestation, when the normal birefringence due to calcium salts did not appear; from days some ears of pallid mice showed a normal layer of calcium granules while others lacked it. The gelatinous layer of the otolith matrix appeared at the normal time, and was normal in amount, position, and staining reactions. The granular matrix was not clearly present at the earliest stages, and days, at which it is normally seen. From days, however, abnormal ears contained free debris which, from its staining reactions, was interpreted as granular matrix. This debris was found in the sacculus and later the endolym-phatic sac. At and days’ gestation ears lacking otoliths showed hydrops of the cochlear duct, sacculus, and endolymphatic duct. Possible mechanisms for the failure of calcification and for the hydrops are discussed. All ears of pallid mice lacked pigment from the secretory regions. The ears of six other mouse mutants, some affecting pigment, and others affecting hearing and behaviour but not pigment, were examined for pigmenta-tion. The pigment of the ear paralleled that of the eyes, and there was no correla-tion between lack of ear pigment and presence of behaviour defects.