We studied development and sexual dimorphism in external measurements of Dall’s porpoises (Phocoenoides dalli) from the northwestern North Pacific. Relative rates of increase in the measurements of the head and flipper decrease postnatally, while those of the flukes, dorsal fin, and girths in the posterior part of the body do not. Growth patterns of the flukes and increase in girth at the anus are different from those in other small odontocetes. This is considered to be related to the fast swimming and presumed deep diving behavior of Dall’s porpoise. Growth rates of appendages decrease with time after birth, as in other cetaceans. Secondary sexual features of males appear in the dorsal fin, flukes, and girth at the anus; the flukes show more distinct sexual dimorphism than the dorsal fin.