An experiment explored the effect of group size on student learning when small groups of students worked together at a computer assisted instruction (CAI) terminal. Both learning effectiveness and efficiency were considered. Sixty college students were divided into three groups--19 students studying alone, 16 studying in pairs, and 15 studying in groups of 3 or 4. Students worked together at the PLATO IV CAI terminal and then completed criterion-based tests individually on the material covered and an attitude questionnaire. No significant differences in achievement were found, but very significant differences in the time to complete the module were observed. Pairs of students required the most time, and groups of 3 or 4 required the least time. When total achievement scores for the group were divided by the total time at the terminal, learning efficiency was found to increase with group size. It may be concluded, therefore, that learning effectiveness is not substantially altered by small group use of CAI terminals, but the relative efficiency is considerably improved. (DGC)