Earlier research has shown that prior knowledge of psychology is positively associated with course achievement. But are these effects attributable to preexisting differences in general ability or aptitude? The authors administered 2 pretests to 353 students early in an introductory psychology course and obtained measures of general student aptitude (i.e., ACT scores), subsequent course participation, and exam performance. In regression analyses, the pretest of psychological knowledge uniquely predicted significant variance in exam scores even with the influences of ACT scores and course participation controlled. A second pretest judging the accuracy of everyday psychological concepts also positively correlated with exam performance but did not predict unique variance in the regression. Thus, beyond general ability, domain-specific prior knowledge facilitates student learning in introductory psychology.