Ecological behaviour is often conceptualized as an instance of cooperating in a social dilemma situation. Thus, it has been argued to relate to dispositional tendencies of moral virtue and pro-social orientation. To embed such notions in models of basic personality, we herein predicted that the recently proposed sixth basic personality factor, Honesty–Humility — which specifically pertains to individual differences in cooperativeness — is linked to environmental attitudes and ecological behaviour. Results from two studies (N = 137 and N = 531, respectively) supported these hypotheses and showed that Honesty–Humility explains incremental variance beyond the remaining, more classical five factors of personality. In addition, mediation analyses revealed that Honesty–Humility exerts part of its influence via individual differences in pro-social value orientations. Individual tendencies to cooperate in social dilemma situations could thus be shown to form a bridge between basic personality dimensions and ecological behaviour. Copyright © 2012 John Wiley & Sons, Ltd.