I investigate the effects of environmental forces drawn from two organizational theories, institutional and ecological theories, on the founding rate of four-year women's colleges in Massachusetts, New York, and Pennsylvania from 1855 to 1968. Results show that institutional forces measured by the presence of elite women's colleges, the development of Catholic women's colleges, and the enactment of the suffrage amendment increase the women's college founding rate and their inclusion leads to organization density decreasing the founding rate monotonically. In the population of four-year women's colleges studied, I argue that institutional forces are suitable measures of legitimacy, while ecological forces are appropriate measures of competition. Further, the presence of delegiHmation forces, separate from competition forces, suggests the need to consider their effects in future analyses.