The purpose of this study was to test the efficacy of a model of critical thinking ontogeny at the undergraduate levels in a four-year college. Critical thinking ability was considered relative to mental ability, academic performance, and academic preparation for college. Students (N=607) in many disciplinary areas took a battery of tests to determine critical thinking and mental abilities. Multivariate analysis was performed and analysis of variance was used to determine significant heterogeneity of variances among the categoric groups. Upper division undergraduates scored significantly higher in critical thinking ability, mental ability, and academic performance than did lower division undergraduates, while undergraduates did not significantly differ in academic preparation for college, indicating that differences in critical thinking scores were not primarily due to academic attrition. (Author/CP)