The Supreme Court's decision in Plessy v. Ferguson (1896) is infamous for its doctrine of “separate but equal,” which gave constitutional legitimacy to Jim Crow segregation laws. What is less-known about the case is that the appellant Homer Plessy was, by all appearances, a white man. In the language of the Court, his “one-eighth African blood” was “not discernible in him.” This article analyzes Plessy as a story of racial “passing.” The existence of growing interracial populations in the nineteenth century created difficulties for legislation designed to enforce the separation of the races. Courts were increasingly called upon to determine the racial identity of particular individuals. Seen as a judicial response to racial ambiguity, Plessy demonstrates the law's role not only in the treatment of racial groups, but also in the construction and maintenance of racial categories.