Even though female food acquisition is an area of considerable interest in hunter-gatherer research, the ecological determinants of women's economic decisions in these populations are still poorly understood. The literature on female foraging behavior indicates that there is considerable variation within and across foraging societies in the amount of time that women spend foraging and in the amount and types of food that they acquire. It is possible that this heterogeneity reflects variation in the trade-offs between time spent in food acquisition and child care activities that women face in different groups of hunter-gatherers. In this paper we discuss the fitness trade-offs between food acquisition and child care that Hiwi and Ache women foragers might face. Multiple regression analyses show that in both populations the daily food acquisition of a woman's spouse is negatively related to female foraging effort. In addition, nursing mothers spend less time foraging and acquire less food than do nonnursing women. As the number of dependents that a woman has increases, however, women also increase foraging time and the amount of food they acquire. Some interesting exceptions to these general trends are as follows: (a) differences in foraging effort between nursing and nonnursing women are less pronounced when fruits and roots are in season than in other seasons of the year; (b) foraging return rates decrease for Ache women as their numbers of dependents increase; and (c) among Ache women, the positive effect of number of dependents on foraging behavior is less pronounced when fruits are in season than at other times of the year. Lastly, in the Hiwi sample we found that postreproductive women work considerably harder than women of reproductive age in the root season but not in other seasons of the year. We discuss how ecological variation in constraints, the number of health insults to children that Hiwi and Ache mothers can avoid, and the fitness benefits they can gain from spending time in food acquisition and child care might account for differences and similarities in the foraging behaviors of subgroups of Hiwi and Ache mothers across different seasons of the year. Valid tests of the explanations we propose will require considerable effort to measure the relationship between maternal food acquisition, child care, and adverse health outcomes in offspring.