Using the nationally representative High School Longitudinal Study of 2009 (HSLS:09), this study documents that rural and small-town students were significantly less likely to enroll in postsecondary STEM (science, technology, engineering, and mathematics) degree programs, compared with their suburban peers. This study also shows that schools attended by rural and small-town students offered limited access to advanced coursework and extracurricular programs in STEM and had lower STEM teaching capacity. Those opportunities to learn in STEM were linked to the widening geographic gaps in STEM academic preparation. Overall, our findings suggest that during high school rural and small-town students shifted away from STEM fields and that geographic disparities in postsecondary STEM participation were largely explained by students' demographics and precollege STEM career aspirations and academic preparation.