This study links the social capital literature with research on student mobility to investigate low test score performance among Mexican origin youth. Specifically, it examines whether Mexican Americans learn less in school than non-Latino Whites, in part because they have limited social capital due to the fact that they are more mobile during their school careers. This study also considers whether different forms of peer social capital, like different kinds of currency, have differential exchange value, and if such differences influence the test-score gap. Findings encourage greater sensitivity to inter- and intra-ethnic distinctions in the socialization process that contribute to group differences in the availability and utility of the resources that inhere in social networks. (Contains 3 tables and 22 endnotes.)