Reported is a study designed to determine the effects of diagnostic testing followed by prescribed remediation on the immediate and retained science achievement of middle school students, and to determine if effects of treatment were consistent across students' race and locus of control (LOC) levels. Three intact seventh-grade science classes were randomly assigned to the following treatment levels: (1) Comparison levels - objectives only; (2) Experimental level I - diagnostic level; and (3) Experimental level II - diagnostic/remediation level. Data were analyzed using ANOVA procedures in a 3x2x2 (treatment x LOC x race) factorial design. Pretreatment achievement data were used as the covariates. The study indicates that students who experienced diagnostic remedial instruction showed a 10% - 12% greater retention of science concepts. Internal LOC students scored higher than did external students. White students showed greater retention than did black students, but these differences were reduced through the use of diagnostic remedial instruction. (Author/GA)