The influence of locus of control and diagnostic testing with prescribed remediation on immediate and retained science achievement was the focus of the study. To test the generalizability of the results across instructional units and to check on the existence of a treatment warm-up period, the experiment was conducted across three biology units. The results supported the existence of the warm-up period with no significant effects of diagnosis and remediation during the first unit. On later units and on the retention measure, the experimental subjects achieved significantly more than the control. No locus of control X treatment interactions were found; although, in one unit, the internal locus of control subjects achieved significantly higher than externals.