This paper analyzes the socioeconomic stratification of achievement in the Chilean voucher system using a census of 4th and 8th graders, a multilevel methodology, and accounting for unobserved selectivity into school sector. Findings indicate that the association between the school's aggregate family socioeconomic status (SES) and test scores is much greater in the private-voucher sector than in the public one, resulting in marked socioeconomic stratification of test scores in the Chilean voucher system. We also find that the amount of tuition fees paid by parents in private-voucher schools has no bearing on test scores, after controlling for the socioeconomic makeup of the school. Implications of these findings for educational inequality in the context of a universal voucher system are discussed. (Contains 10 tables.)