Research has shown that rural high school students in the United States (and elsewhere) have lower academic achievement than their nonrural counterparts. The evidence for why this inequality exists is unclear, however. The present study takes up this issue with a narrowing of the focus. Using the database of the Educational Longitudinal Study of 2002-2004, the author investigates reasons for the rural achievement gap in mathematics during the last 2 years of high school. His approach focuses on the geographic disparities in the opportunity to learn advanced math. The findings show that geographic variation in high school resources and practices—operationalized as the availability of advanced math courses, evidence for track assignment by family background, and the quality of instruction—do not account for the rural math achievement gap. On the other hand, geographic variations in the opportunity to learn that result from differences in family socioeconomic status (SES) and the influence of friends’ academic commitments and aspirations do help to explain why rural high school students learn less mathematics than their nonrural counterparts. The observed effects on math achievement by family SES and friends are in part direct effects, but these factors also have critically important indirect effects because they appear to influence student motivation to take advanced math courses.