This cross-sectional study examined the associations among perceived vulnerability to disease, aging knowledge, and ageism (positive and negative) in a sample of undergraduate students enrolled in a human development course (N = 649; M age = 19.94 years, SD = 2.84 years). Perceived vulnerability to disease and aging knowledge were associated with self-reported ageist behaviors. Undergraduates who viewed themselves as more susceptible to disease and knew less about the aging process tended to report more negative ageist behavior. Sex moderated the association between aging knowledge and ageist behavior, indicating the association was stronger for males. Discussion focuses on implications for gerontology educators. (Contains 5 tables and 1 figure.)