This study explored the importance of group interaction for learning. It investigated: (1) the relationship between group interaction and achievement, and whether this relationship was constant across ability levels; and (2) the importance of ability, extroversion-introversion, and group composition as predictors of group interaction. The sample consisted of 96 students in grades 7, 8, and 9 from two average and two above-average general mathematics classes. The topic of the instructional materials was consumer mathematics. Instrumentation used in the study consisted of an achievement test, personality measure, and an observation instrument. The analysis of achievement, interaction in the group, and student and group characteristics revealed three important findings. First, asking a question and receiving no answer was the predominant group interaction variable predicting achievement, with receiving no answers detrimental. Second, the amount of time spent working with other students influenced achievement, but the relationship differed across ability levels. Working with others was beneficial for low ability students but somewhat detrimental for high ability students. Third, the interaction variable asking a question and receiving no answer was predicted from group composition and extroversion-introversion, suggesting that group interaction mediates the effects of some individual and group characteristics on achievement. Further studies with carefully defined combinations of group composition, ability, personality, interaction, and achievement are suggested. (MP)