Each member of 7 good and 5 poor high school basketball teams described himself on a self-description questionnaire and then predicted how his most preferred and least preferred co-workers would describe themselves on the questionnaires. The measures obtained for study included: the assumed similarity of each member to his preferred co-worker (ASp); assumed similarity to his rejected co-worker (ASn); and the assumed similarity between the preferred and rejected co-workers of each member (ASo). The ASo scores of the most preferred co-worker of the team correlated negatively with the team effectiveness. The scores of other team members were not reliable predictors of team effectiveness.