This article analyses the founding rate of nationally active homosexual rights interest groups in the United States for the period 1950–98. Drawing upon the extensive organizational ecology literature, we test the hypothesis that the founding rate of homosexual rights interest groups is related non-monotonically to the number of groups in the population. Our statistical analyses support the hypothesis that as population density rises from very low to high, the founding rate first rises but eventually decreases. This pattern holds when we control for a number of contextual variables. In all, the data provide a great deal of support for the theory of density dependence – a novel approach to the study of interest group formation.