The purpose of this study was to examine for the middle grades how students' cognitive reasoning level contributes to the variability of their achievement test performance. While previous investigations have indicated that measures of reasoning development are associated with measures of achievement, few such studies have attempted to control for differences in scholastic aptitude. In this particular investigation, the performance of 213 middle grades students on a test of formal reasoning was analyzed in relationship to scores on tests of achievement and scholastic aptitude. Results support cognitive reasoning as a significant determinant of achievement test performance when considered alone but not when controlling for scholastic aptitude. Results suggest that performance on reasoning tests is related to performance on achievement tests in a fashion very similar to performance on scholastic aptitude tests. Results further encourage middle grades educators to consider students' levels of reasoning when planning instruction. (Author/JAZ)