In the domain of travel mode choice behavior, the interaction between ecological norm orientation and the external aspects "fare" and "subway station range" was investigated in an experimental field study. The ecological norm orientation is conceptualized based on the Schwartz theory on altruistic behavior, which is then applied to the environmental context. In a random sample of 160 persons, fare was experimentally manipulated by distributing free public transport tickets, whereas the station range was varied by selecting test participants at different distances from a station. Within the norm activation model, the mobility-specific personal ecological norm proves to be the strongest predictor of travel mode choice as recorded in standardized questionnaires. Reducing the fare by distributing free tickets has a quantitatively similar effect. The results suggest that the "economy-plus-moral" formula best describes the fact that the integrative mechanism (external factor fare plus normative ecological orientation) is the determinant of travel mode choice.