We examined geographic variation in external measurements of Dall’s porpoises, using 269 sexually mature specimens from throughout their range, excluding coastal waters off North America and the northern Okhotsk Sea. The lateral white patch is more caudad in dalli-type porpoises from the Sea
of Japan than in those from other areas. This is a distinguishing character of the Sea of Japan–Okhotsk Sea population, and it indicates its independence along with genetic, morphological and ecological differences previously reported. Morphological and genetic differentiation suggests an older origin for this population. Sexual dimorphism is less developed in mature males from the eastern North Pacific than from other areas.