In this experiment designed to trace the development of rivalry in Anglo American and rural Mexican children, rivalry (behavior intended to lower the outcomes of a peer) was measured by 4 choice conditions wherein the rivalrous alternative in all conditions was the choice that left fewer rewards (marbles) to the chooser's peer. After all choices were made, the children could trade the marbles for toys. The conditions, designed to systematically vary absolute and relative outcomes, were presented to Anglo American and Mexican children of ages 5-6 and 8-10 from 2 settings: Los Angeles, California, and Nuevo San Vicente, Baja California, Mexico. Forty-eight children from each culture, 6 boy pairs and 6 girl pairs, participated as subjects. It was revealed that older children were significantly more rivalrous than younger children; Anglo children were significantly more rivalrous than Mexican children; and the cultural differences tended to increase with age. The effect of conditions was significant, indicating that for all groups rivalry was greatest when accompanied by both relative and absolute gains. The opportunity to avoid a small relative loss increased rivalry more than opportunity to accrue a small absolute gain. The development with age of greater rivalry in boys than girls was present for the Anglo but not Mexican children. (Author/NQ)