Abstract We analyze the relationship between legitimation, resource mobilization, and political opportunity and the founding rate of national environmental organizations between 1895 and 1994. We address recent criticisms that organizational ecologists' reliance on the density dependence model—which treats legitimation as an unmeasured intervening variable—has failed to capture the active sociopolitical character of this process. We advocate a more historical approach to legitimation which relies on print media to construct direct measures of legitimation. Specifically, we use the yearly count of environmental books published as a measure of the legitimacy of environmentalism in the U.S. This count remains statistically significant and positively related to foundings even when other variables are added to the model. A negative second-order effect of total books suggests that the recent proliferation of state and local environmental organizations is beginning to depress the founding rate of national organizations. Variations in resource mobilization and political opportunity also have affected foundings, though variables measuring the latter were not significant in the full model.