This study investigated aptitude-treatment interactions (ATI) in fourth, fifth, and sixth grade students' learning in large and small group teaching approaches. Each of two elementary teachers taught a two-week fractions unit to two classes of randomly assigned students. Each class received one approach. Students completed aptitude measures at the beginning of the study, and achievement, attitude, and retention measures at the end. Regression analyses showed significant ATI for preference for approach and for ability. Students who said they preferred small groups did worse in that approach than in the large-group approach. High ability students performed better and had more positive attitudes in the small group approach, and low ability students performed better and had more positive attitudes in the large group approach. High ability students benefited by "teaching" their peers in the small group. (Author/MP)