Reported is a study that explores the effect on student achievement of diagnostic-prescriptive instructional strategies on preservice elementary education majors (N=43) enrolled in an introductory biology course. Factors of pre-treatment achievement and locus of control were analyzed as well. Units on Mendelian genetics, modern genetics, and evolution were taught to two randomly chosen groups of students. Treatments were identical for the two groups except for the diagnostic-prescriptive materials being tested. Data analysis indicates that achievement increased as students in the diagnostic-prescriptive treatment completed the three units and a final examination. Locus of control was shown to be of minimal importance in determining student achievement. (CS)