Some growth parameters of the western North Pacific dalli-type Dall’s porpoise were estimated from 167 specimens taken with hand-held harpoons on two cruises and compared with data obtained previously from porpoises killed incidentally in the salmon gill net fishery. These parameters were compared with previously published figures for the Japanese coastal population. Individuals from the Pacific coast of Japan (96% were truei-type) attained sexual maturity at body lengths 12 to 17cm larger than dalli-type individuals in the offshore waters. This is an indication of genetic differentiation between the two populations in addition to the previously reported external pigmentation. The possible usefulness of the growth parameters for the detection of genetic isolation between two major color types off the Pacific coast of Japan is indicated.