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.