Immigration is commonly considered to be selective of more able individuals. Studies comparing the educational attainment of Mexican immigrants in the United States to that of the Mexican resident population support this characterization. Upward educational-attainment biases in both coverage and measurement, however, may be substantial in U.S. data sources. Moreover, differences in educational attainment by place size are very large within Mexico, and U.S. data sources provide no information on immigrants’ places of origin within Mexico. To address these problems, we use multiple sources of nationally-representative Mexican survey data to re-evaluate the educational selectivity of working-age Mexican migrants to the United States over the 1990s and 2000s. We document disproportionately rural and small-urban-area origins of Mexican migrants and a steep positive gradient of educational attainment by place size. We show that together these conditions induced strongly negative educational selection of Mexican migrants throughout the 1990s and 2000s. We interpret this finding as consistent with low returns to the education of unauthorized migrants and few opportunities for authorized migration.