Competitive or cooperative behaviors of children were observed in two experiments. In Experiment 1, 7- to 9-year-old boys and girls played a board game with same- or opposite-sex partners under instructions which stressed either group or individual performance. Girls' behavior varied as a function of both instructions and partner sex. Girls playing the game with boys showed more highly competitive behavior than did girls who played the game with girls. Boys were highly competitive under all conditions. In Experiment 2, preschool children were observed in a similar game. For both sexes, children who played the game with opposite-sex partners were more competitive and obtained fewer rewards than those who interacted with same-sex partners. Thus, with development, boys show a change from selective competitiveness to a very general tendency to compete, while girls show a consistent tendency to vary their game behavior according to situational cues such as those provided by instructions or the sex of a game partner.