PIP: This study suggests that the degree to which work and childcare are competing uses of time is an important determinant of the female labor force participation (FLFP) fertility relationships in less developed countries. A negative relationship may be found where work and childcare are competing uses of time, and no relationship (or even a positive relationship) where they are not. Several measures of competing time use are developed. The competing time use hypothesis is tested using sample survey data from Mexico City. Focus is on correlation, not causation. A basic assumption is that FLFP and fertility decisions are made jointly and depend on income, prices, tastes, wages, employment opportunities, and current family size. The empirical work deals with the direction and significance of partial correlations between FLFP and fertility and whether these correlations vary predictably according to the degree to which work and childcare are competing uses of time. The data were drawn from a multistage, stratified, clustered probability sample of married Mexican women living in the Mexico City Metropolitan Area. All women were living with their husbands at the time of the interview in early 1971. Sample size was 798. The empirical results provide strong support for the hypothesis that a negative FLFP fertility relationship will exist only if work and childcare are competing uses of time. When no distinction was made regarding competing time use, there was no significant FLFP fertility relationship. Women who had worked at some time since marriage had very similar levels of fertility to women who had not. When distinctions regarding competing time use were made, large fertility differences were evident. Workers for whom market work and childcare cannot be performed simultaneously had significantly lower fertility than nonworkers; workers for whom market work and childcare could be performed simultaneously had similar or higher fertility than nonworkers. Full time workers had significantly lower fertility than nonworkers, but part-time workers did not. The modern sector workers had significantly lower fertility than nonworkers but traditional sector workers did not. Policies that deal with female employment in developing countries need to focus on both levels of employment and the nature of that employment.^ieng