Field observations were collected to assess the social behaviour and foraging strategies of free-ranging killer whales from the eastern North Pacific (near Vancouver Island, Canada) and the western South Atlantic (near Peninsula Valdez, Argentina). The Vancouver Island study concentrated on the environmental correlates of group size and the behavioural dynamics of social groups. There were no correlations between foraging behaviour and small-scale habitat use, however both group size and the spatial distribution of groups were correlated with foraging behaviour. In Argentina the subject whales intentionally stranded to capture pup sea lions. It was possible to observe details of prey choice and foraging strategy. Three social groups were observed in the study area. Area use suggested that the different groups were employing different strategies. Whales within social groups shared prey, but one group would exclude another from the best hunting areas. Whales invested the greatest effort in the area of highest yield, and on the prey-type that required the least effort to catch. Energetic calculations suggested that the rate at which these whales captured sea lion prey was just sufficient to sustain them. Two genetic components, the hypervariable 'minisatellite' loci, and the mitochondrial genome were investigated for each study population. In addition, further samples from Iceland and other populations near the sites at Peninsula Valdez and Vancouver Island were analysed. Whales within social groups at Peninsula Valdez were more closely related than between social groups. In general, whales within local populations had very high levels of genetic similarity compared to between population comparisons. This implies inbreeding within and genetic isolation between populations. Two genetically isolated populations (both near Vancouver Island) were sympatric, and the degree of genetic isolation was equal to the level seen for comparisons between the Atlantic and Pacific. A hypothesis is presented on the role of resource exploitation in the structuring of social groups, and the consequences for the genetic structuring of populations.