Precollege schools use their computers predominantly for drill and practice in arithmetic. It has been shown by research to be the most effective application of CAI (Computer-Assisted Instruction). The present study used naturalistic methods to examine differential effectiveness of CAI in arithmetic for high- and low-achieving students. Seven elementary school students with varied backgrounds with regard to socioeconomic status (SES), grade level, type of school, and achievements in arithmetic participated in the study. The study identifies strategies that students use to advance effectively through the levels of the CAI practice with material not yet studied in class. The study also discusses software- and hardware-related errors that students make while working with the CAI system. In many cases, these errors are unrelated to the knowledge of arithmetic. It seems that the low achievers, more than the high achievers, are prone to make these types of errors, which are partly responsible for ineffective CAI work. The study concludes that the higher achieving students observed were much more able than the observed low achievers to adjust to the special environment of computer work and to derive great benefits from it.