The persistence and size of social groups can be plastic and governed by ecological selection or be under greater genetic control and constrained by phylogenetic inertia. Comparing sociality of phylogenetically divergent populations under the same ecolog-ical conditions or between groups within a population under different ecological conditions can identify the relative influence of ecological selection on group formation. Here, we compare the size and persistence of social groups within a community of Atlantic killer whales, comparing between data collected from an area around Scotland where the whales have mainly been seen to hunt seals and data collected from an area around Iceland where the whales have mainly been seen to hunt herring. Additionally, we compare the observed social structure with that of previously studied Pacific ecotypes. Atlantic killer whale groups in both locations had a stable long-term primary social tier (association index level . 0.8) similar to that of Pacific killer whales. However, associations between these groups were much lower when hunting for seals than for fish in both the Atlantic and the Pacific. The occurrence of these differences in sociality between Atlantic groups, which are linked in a single social network, suggests that ecological selection partially determines sociality in this species. Furthermore, if sociality was constrained by phylogenetic inertia, then the Atlantic killer whales would all be expected to be more similar to the Pacific fish-eating ecotype than the more phylogenetically distant Pacific mammal-eating ecotype. Our study suggests that sociality in killer whales is to some extent plastic and can be adapted to the local ecological conditions.