A 1-year study of the intergroup behavior of rhesus monkeys (Macaca mulatta) on Cayo Santiago, Puerto Rico, was begun in September 1968. Intergroup contacts occurred between entire groups, parts of groups, and single individuals and varied in intensity from violent chases and squabbling to play, matings, and other social behaviors. A series of analytical categories based on the number of individuals participating in the interaction and the nature of the interaction, e.g. aggressive, nonaggressive, sexual, are proposed and discussed. Rates of aggressive group-level interaction did not change significantly between the mating and nonmating season, while wounding, branch-shaking, and male stalks show a monthly distribution with peaks during transition months between seasons. Aggressive intergroup interactions were most frequent from 08.00-09.00 h with a lesser peak from 15.00-16.00 h. Participation in intergroup behaviors by the members of a single social group, group F, is analyzed and an attempt made to infer the ontogeny of participation in intergroup behaviors, thereby relating observed behavior to the life cycles of individuals.