This study investigated: (1) the differences in competition-cooperation behavior of male and female children in a two-person game; and (2) whether sex differences in behavior were affected by either the sex of the game partner or by instructions. Subjects were 72 urban, middle class boys and girls. Their mean age was 7 years 11 months. Children were tested in pair groupings which included included 12 boy-boy, 12 girl-girl and 12 boy-girl pairs. The object of the game used in the behavior evaluation was to move a marker from the middle of the game board to either goal. Each partner could move only one place at a time: towards his own goal (competitive moves), his partner's goal (cooperative moves) or sideways (neutral moves). After each pair had completed six trials of the game, instructions were given which either stressed group orientation and deemphasized competitiveness or stressed individual orientation and competitiveness. Twelve additional trial games were then played. Results indicated that girls show a greater flexibility of behavior as a function of sex of partner or of instructions than boys do. In comparison to girls, boys show constant highly competitive behavior. (SDH)