The purpose of this study was to investigate the effects of the use of hand-held calculators on the teaching of decimal arithmetic in seventh and eighth grade pre-science units. In particular, the effects of calculators on pencil and paper computational skills and on conceptual understanding were studied. The study consisted of a pre-unit evaluation, an 8-day learning unit with a series of worksheets designed to present the concepts of decimals, and a post-unit evaluation. The subjects were 384 seventh and eighth grade students of Brookings (South Dakota) Middle School, divided so that only one-half of each grade would use calculators during the instructional period. Neither group used calculators on either the pretest or posttest. Analysis of variance and analysis of covariance revealed no significant differences between the two groups on either the pretest or the posttest. The analysis indicates that calculators, as instructional aids, did not promote or hamper the performance of the students on the decimal operations and that the computational skills of the students were not dependent on the machines. (Author/MLF)