Geographic variation was observed in skulls of several otariid species, with a general change in size corresponding with a change in latitude and primary productivity. The largest specimens were from cool temperate localities, conforming mostly to Rensch's rule. Skulls of Australian sea lions from Western Australia were generally smaller in condylobasal length, but were more robust than those from South Australia. The subantarctic fur seal did not conform to Bergmann's rule: skulls from Amsterdam Island (37°55´S) were largest, those from Gough Island (40°20´S) intermediate and those from Marion Island (46°55´S) the smallest. For both sexes, skulls of southern sea lions from the Falkland Islands were smaller than their equivalents from mainland South America. Similarly, skulls of South African fur seals from south-east South Africa appeared smaller than those from the west coast of South Africa and Namibia; skulls from Namibia grouped separately from those of south-east and west coast, South Africa. We postulate that the Otariidae are in the process of species divergence, much of which may be driven by local factors, particularly latitude and resources.