Self-categorization theory suggests that inter-group comparisons inform individual behaviour by affecting perceived in-group stereotypes that are internalized by group members. The present paper provides evidence for this chain of effects in the domain of environmental behaviour. In two studies, inter-group comparative context was manipulated. Study 1 found that the perceived in-group stereotype, self-stereotype (as represented by the reported value centrality), and behavioural intentions shifted away from a comparison out-group (irrespective of whether this was an upward or downward comparison). Study 1 also revealed that the effect of comparative context on individual environmental intentions was mediated by the perceived in-group stereotype and by changes in personal values. Study 2 extrapolated the observed effect on actual behavioural choices. The findings demonstrate the utility of a self-categorization approach to individual behaviour change.