By using skulls of sperm whales (Physeter macrocephalus), we analyzed the relative growth of skull length in relation to that of body length. We also analyzed the growth of various parts of the skull in relation to skull length. We used data of 3 males and 1 female collected off the coast of Japan and those of 4 males reported previously. In males, the proportion of skull length to body length increased with growth from 26% (body length, 10 m) to 32% (body length, over 16 m, most of which were considered as physically mature). In contrast, the proportion reached less than 25% in the physically mature female. In males, the relative length of the rostrum and the mandible to skull length increased with growth, and the posterior margin of the occipital bone tended to protrude posteriorly. This posterior protrusion could be considered as a secondary sexual dimorphism observed only in the skull of physically mature males. These characteristics would be associated with sex differences and deep diving as well as to the life style and breeding strategy of sperm whales, a species in which males physically compete for breeding rights.