Educational psychologists find that prior knowledge influences new learning. We examined whether course achievement for introductory psychology students is facilitated or impaired by their prior knowledge of psychology. We administered a pretest exam to 422 students early in the semester and gathered subsequent exam scores and other measures of student achievement. Students generally performed poorly on the pretest, as expected, but regressions revealed that pretest scores were uniquely positive, significant predictors of student achievement with other influences on achievement (e.g., homework, attendance) controlled. Further analyses suggested that prior knowledge is a significant resource that faculty can enlist in their instruction.