This paper investigates the formation and evolution of friendship cliques among preadolescent youth in elementary and junior high grades 4 through 8. Two sets of data were collected: the first set consisted of cross sectional data from 51 classes (grades 5 through 8); the second set contained sociometric data collected from 11 classes (grades 4 through 6) at several time points over a school year. Students' assignment of best friend designations to classmates were used in the analysis. Characteristics of individual children and of their classroom settings were found to affect the number of friendships in the class and the number and size of cliques. Class size and classroom organization were found to influence average number of friends and number, size and stability of cliques. Age and sex affected density of the friendship network, number of social isolates and clique membership. While some cross-sex friendships were reported, a total separation by sex existed in the cliques in every class. Students in open classes received fewer friendship choices on the average than those in more structured classes but the choices were more evenly distributed in the open settings. It was concluded that elementary children engage in social behavior that is characterized by a tendency toward cliquing, sex separation and exclusivity of relations. These characteristics are affected by the organizational setting to which the students interact. (Author/RH)