Several recent studies suggest concrete learners make greater gains in student achievement and in cognitive development when receiving concrete instruction than when receiving formal instruction. This study examined the effect of concrete and formal instruction upon reasoning and science achievement of sixth grade students. Four intact classes of sixth grade students were randomly selected into two treatment groups; concrete and formal. The treatments were patterned after the operational definitions published by Schneider and Renner (1980). Pretest and posttest measures were taken on the two dependent variables; reasoning, measured with Lawson's Classroom Test of Formal Reasoning, and science achievement, measured with seven teacher made tests covering the following units in a sixth grade general science curriculum: Chemistry, Physics, Earth Science, Cells, Plants, Animals, and Ecology. Analysis of covariance indicated significantly higher levels (better than 0.05 and in some cases 0.01) of performance in science achievement and cognitive development favoring the concrete instruction group and a significant gender effect favoring males.