Sociologists often assert, but rarely test, the claim that organizational diversity benefits social movements by invigorating movement vitality and facilitating success. Our analysis of black civil rights organizations shows that goal and tactical diversity of a social movement is largely a function of organizational density, level of resources available to the movement, and the number of protests initiated by the movement. Goal diversity increases the rate of protest, whereas tactical diversity increases the likelihood of achieving a desired policy outcome. These findings advance our understanding of social movements and organizations by illuminating how organizational dynamics of a social movement might change over time, and in turn how this change might affect the vitality and desired outcomes of social movements. (Contains 4 tables, 2 figures and 8 notes.)