This study investigated how measures of both crystallized cross-domain knowledge and fluid relational reasoning abilities contribute to high-school students’ scholastic aptitude and content-specific achievement. The participants for this study were 211 ninth-grade and 76 twelfth-grade students enrolled in an all-male parochial high school. A series of multivariate multiple linear regression tests were conducted to examine the ability of the three crystallized cross-domain knowledge predictor variables (i.e., Language Skills, Mathematics, Reading Comprehension) and fluid reasoning ability predictor variable (Test of Relational Reasoning - TORR) to predict performance of ninth-grade students’ scholastic aptitude (Preliminary Scholastic Assessment Test - PSAT), and content-specific achievement (Social Studies, Algebra, and Spanish final examinations); and performance of twelfth-grade students’ content-specific achievement (Advanced Placement Psychology examination). Results of these analyses revealed that fluid relational reasoning was the strongest unique predictor of performance for ninth-grade students on the PSAT, Algebra and Spanish final examinations, and for twelfth-grade students on the AP Psychology examination. Crystallized cross-domain was found to be not as strong of a predictor as fluid relational reasoning on the five outcome measures. Results from this study suggest that students who have greater fluid relational reasoning abilities may perform better on these assessments. The research also includes delimitations, practical limitations for educators, and suggestions for future research to expand the scope of this study.