How does the appearance of a new immigrant group affect the integration of earlier generations of migrants? We study this question in the context of the first Great Migration (1915-1930), when 1.5 million African Americans moved from the US South to northern urban centers, where 30 million Europeans had arrived since 1850. We exploit plausibly exogenous variation induced by the interaction between 1900 settlements of southern-born blacks in northern cities and state-level outmigration from the US South after 1910. Black arrivals increased both the effort exerted by immigrants to assimilate and their eventual Americanization. These average effects mask substantial heterogeneity: while initially less integrated groups (i.e. Southern and Eastern Europeans) exerted more assimilation effort, assimilation success was larger for those culturally closer to native whites (i.e. Western and Northern Europeans). Labor market outcomes do not display similar heterogeneity, suggesting that these patterns cannot be entirely explained by economic forces. Our findings are instead more consistent with a framework in which changing perceptions of outgroup distance among native whites lowered the barriers to the assimilation of white immigrants.