The number of interdisciplinary, degree-granting fields in American colleges and universities has grown rapidly, with socially incorporative programs (women's studies, ethnic studies, and non-Western area studies) accounting for a large share. These fields have diffused widely over time, while other interdisciplinary fields have not. Variables based on ecological features of academic organizations provide a better guide to the location of interdisciplinary fields than variables based on student demographic composition or organizational stratification. Most studies of interdisciplinary change have focused on external resource providers and new technology development, but this study suggests that social movements are another important source. (Contains 1 figure, 5 tables and 9 footnotes.)