Highly significant (α = 0.01) differences in analysis of 473 skulls of harbour porpoises indicate separate eastern Pacific, western Atlantic, and eastern Atlantic populations. Including the isolated population in the Black Sea—Sea of Azov, four major populations in this species are presently definable. Small sample sizes and incomplete data prevented conclusive tests for significant differences in subpopulations within these, but there are trends which suggest some segregation of Dutch, Baltic, and eastern English (North Sea) animals might be demonstrated with large samples. Skulls of western Atlantic harbour porpoises are larger than those of eastern Atlantic animals; skulls of eastern Pacific porpoises are smaller yet. This size order is more strongly marked in males than females. Despite their smallest overall dimensions, Pacific harbour porpoise skulls are most attenuate, with significantly longer jaws and more teeth than Atlantic animals. Females exceed males in size in each population. The configuration of dentinal growth layers was consistent in specimens from the Bay of Fundy, the Baltic, North Sea, and north-west Africa, lending support to the hypothesis of endogenous control of layering, rather than exogenous control through seasonal food availability.