This article discusses how disputes over church and state in modern America have been affected by societal attitudes toward Roman Catholicism. It is part of a book in process, a legal and cultural history of church and state in America since the 1940s, when the Supreme Court regularly began to decide such disputes. From the late 1940s through the 1970s, a fairly strict separation of church and state became the dominant ideal in Supreme Court decisions and in elite culture. But the separation ideal has declined in influence in recent years, although it remains strong in some respects. This article discusses how attitudes toward Roman Catholicism in America fit into this story, in three different periods. First, the late 1940s and early 1950s saw a resurgence in fear and distrust of Catholicism, and these contributed to the rise of church-state separationism in constitutional decisions. Second, in the 1960s and 1970s, negative attitudes toward Catholicism might have been expected to decline, but they lingered, and for that and other reasons the ideal of church-state separation became even more strict, especially in restricting aid to religious schools. Finally, in the 1980s and 1990s, the partial rollback of separationism, especially of its restrictions on parochial school aid, has been partly influenced by changes in the nature and importance of anti-Catholicism. Although negative attitudes toward Catholicism certainly remain significant, they are less widely held, are less focused on Catholic schools as such, and are only part of a broader distrust of politically active social conservatives, including evangelical Protestants. The decline in the centrality of the "Catholic question" has contributed to a shift in the focus of church-state ideals away from church-state separation and toward equal treatment of different religious views.