This study was designed to determine experimentally the relationship between achievement and grade level for 64 tenth-grade students, 48 eleventh-grade students, and 95 twelfth-grade physics students. All students were enrolled in the Project Physics course in their respective schools. The Physics Achievement Test (PAT) and the algebra I and II parts of the Cooperative Mathematics Test were administered at the beginning of the year. Posttest scores were derived from the average of the first four unit tests developed for the Project Physics course. The multiple linear regression approach to analysis of variance was used to test the null hypothesis that there would be no statistically significant relationships. A stepwise regression procedure was used to select the best set of predictor variables. These variables selected were used in computations relating to analysis of covariance. Results showed that tenth-grade students scored significantly lower on the posttest than each of the other grades. The eleventh-grade students were the superior group on both posttest and the I.Q. measures. The two best predictors of achievement were the algebra test and Physics Achievement Test. (Author/EB)