This study assessed the effect of group size and presentation mode on acquisition of the process skill of forming and testing hypotheses and selected attitudes. Preservice elementary teachers (N=87) in five intact classes received two 1-hour sessions presented by microcomputer, 1 week apart. Half of the subjects were assigned to three-member cooperative learning groups; the rest worked alone at the computer. Matched problem situations which required the forming, testing, and revision of hypotheses were presented as color simulations to one-half of the subjects and as exercises to the rest. The effect of group size and mode of presentation was found to depend on subjects' initial hypothesizing skills and formal reasoning ability. Subjects with low formal reasoning skills benefitted more from small group interaction than from working alone. Subjects with higher formal reasoning ability also performed better after receiving textual exercises than similar subjects who interacted with color computer simulation. Also, cooperative group learning and color computer simulation were more effective than individual learning and computer text in elevating learners' perceived success in using the computer and attitude toward the program being used. It is suggested that pre-assessment of learner aptitudes is needed before prescribing a mode of computer presentation or group size. (Author/JN)