Previous investigations of the effects of field dependence-independence or the level of operational development on the mathematics achievement of children in the lower elementary school grades have involved the administration of concrete operational tasks (e.g., classification, conservation, and seriation). The present study was designed to examine the influence of these factors on the mathematics achievement of sixth, seventh, and eighth graders by using formal operational tasks (i.e., combinations, propositional logic, and proportionality). Results were analyzed using total mathematics achievement test scores as well as scores on subtests of computation, concepts, and problem solving. Field-independent students scored significantly higher than field-dependent students on the total mathematics, concepts, and problem-solving tests. High-operational students scored significantly higher than their low-operational peers on all tests. Educational implications of the findings are discussed.