A new recessive lethal mutation in mice that produces the otocephaly defect is described. The mutation, provisionally named oto is located on chromosome 1, within, or just outside of, a previously existing inversion, In(1)1Rk, and was probably induced by X-irradiation. The penetrance of oto is nearly complete on C57BL strain backgrounds but is reduced to a variable extent on other backgrounds. The previously reported liability to spontaneous otocephaly in the C57BL strains appears to increase the penetrance of oto. Studies of the sequences of developmental changes (conducted primarily by scanning electron microscopy) and of the range of defects indicate that a primary deficiency involving the anterior aspect of the embryonic disc occurs in affected individuals. An hypothesis related to deficiencies in mesodermal populations is presented as the basis for the craniofacial and brain defects observed.