In an experiment conducted in 7 eighth-grade (Ages 13-14) classes in Singapore, the authors evaluated the effects of the group investigation method of cooperative learning versus the effects of the traditional whole-class method of instruction on students' academic achievement and on their motivation to learn. The authors also investigated students' perceptions of group investigation. Students in group investigation and in whole-class instruction advanced to the same extent over the course of the experiment. Neither method was more effective academically than the other method. As expected, the high-achieving students had significantly higher academic achievement than did the low-achieving students. The group investigation method did not have differential effects on the 2 groups of high and low achievers. Group investigation affected high achievers' motivation to learn on the Criteria subscale only. (Contains 7 tables and 1 figure.)