Two hundred and eighty-nine skulls of Dall’s porpoises from the North Pacific Ocean, Sea of Japan, Sea of Okhotsk, and Bering Sea were investigated morphometrically. A clear pattern of geographic variation was found in overall size of the skull. In the North Pacific, overall skull size became gradually smaller eastward from the coast of Japan to the offshore eastern Pacific, and it became larger again off the coast of California. Specimens from the Sea of Japan-Okhotsk and Bering Sea had larger skulls than those from the central North Pacific. The distribution of primary productivity corresponds well with this pattern of geographic variation, suggesting that quantity of food might affect the overall size of Dall’s porpoise skulls. By canonical discriminant analyses, about 70% (male) and 90% (female) of specimens of the Sea of Japan-Okhotsk population were distinguished from those of other populations. This agrees with the results of genetic studies.