An examination was made of what determined women's opportunities to participate in the United States labor force from 1940 to 1978. Using a model drawn from ecological and competition theory, the data examined suggest that the expansion of the economy, the relative proportion of women in the population, female tertiary education, and governmental involvement in the economy operate to increase women's opportunities in the labor force. Unionization and fertility operate to decrease women's opportunities. Empirical results from time-series analysis indicate that most of these factors are significant and have the predicted effects with the exception of the nonsignificant effects of fertility. Also, some labor force differences by female age groups are found. Directions for future research should include the more direct specification of female and male competition in the labor force and the suitability of this model for cross-national analysis of female labor force participation. (Author/KC)