A study was made of the effects of group size and time-on-task on the ability of 259 fifth- and sixth-grade students to learn in the context of computer assisted instruction. Predominantly Jewish, subjects attended one of three schools in an upper-middle-class area of Montreal. After the Basic Word Vocabulary Test and a background/demographic questionnaire were administered as pretests, subjects were randomly assigned to groups differing in size. In the fifth grade sample, there were 22 groups of 4 children, 23 groups of 2, and 22 groups of 1. In the sixth grade sample, there were 15 groups of 4 children, 13 groups of 2, and 12 groups of 1. Time-on-task was distributed across schools. One school received one half-hour treatment, a second received two half-hour treatments, and a third received three half-hour treatments. During the treatment phase, subjects were assigned computers and were told (1) to complete Level 1 of the "Word Attack" software program, and (2) to continue the program at whichever of the three remaining levels they chose. While students worked, experimenters gathered observational data. After completing experimental sessions, subjects were given three post-tests: two vocabulary tests, and one attribution questionnaire. Preliminary findings indicate that improved achievement resulted from increased time-on-task, but not from group size. (RH)