According to Hardin (1968), environmental deterioration stems from self-interest undermining people's resource conservation in the collective interest. Not surprisingly, selfless prosocial motives, such as personal feelings of moral obligation, have often been recognized as a key force behind people's environmentalism. In our research, we anticipated that environmentalists—people with an inclination for pro-environmental engagement—would possess a propensity to generally act prosocially. In an extension of previous work, we expected that a well-established self-report measure of past conservation behavior would predict people's active participation in a psychological experiment. Based on subjects' degree of environmental engagement, originally established in 2003, we re-contacted a sample of 502 persons in 2005. Of these 502 (68.5% low, 31.5% high in environmentalism), 131 showed up for the announced experiment. Among those participants, we found that environmentalists' prosocial personalities were additionally reflected in their social value orientations. Ninety percent of the environmentalists turned out to be prosocials, whereas only 65% of the less environmentally engaged subjects were prosocials. Overall, our findings lend credit to a notion of environmentalism as an indicator of even subtle quantitative differences in a person's prosocial trait level. By and large, environmentalists acted more prosocially even in mundane activities unrelated to environmental conservation. Additional evidence comes from the commons dilemma experiment in which the participants partook. There, we generally found comparatively more cooperation with others for the collective good from people high in environmentalism. Our findings represent circumstantial evidence for a prosocial propensity dimension along which people differ, and which is also reflected in people's pro-environmental behavioral performance. If, however, environmentalism has to be regarded as indicative of a prosocial trait rather than a state-like motive, high hopes for moral norms and other prosocial motives in environmental conservation do not seem warranted.