An experimental task with accompanying apparatus was developed for use in the study of developmental and cultural differences in the cooperative-competitive behavior of children in a small Mexican town and in California. Two groups of 20 Mexican children (aged 7-8 and 10-11), from an elementary school in a town in Baja, California, Mexico, were used; the Anglo American sample included 3 groups of 24 children (aged 4-5, 7-8, and 10-11) from day-care centers in Los Angeles. The children were tested using a 2-person cooperation-competition marble-pull game designed for the study. A pair of children of the same sex and culture was seated, 1 child at each end of the game table; the children then played for a marble placed in a holder. In the "tug-of-war situation," cooperative behavior resulted in 1 child getting a marble while competitive behavior resulted in neither child getting a marble. At the conclusion of 10 trials for each pair of children, informal interviews were held to determine, for children who competed maladaptively (competitively), the extent to which they were aware of possible interacting methods. Two instructional trials followed the interview, and subjects completed a second series of 10 trials. Results indicated a higher level of cooperation among Mexican than among Anglo American children and an increase in nonadaptive competition with age among the Anglo American children. (NQ)