The literature on assimilation and ethnic identity formation largely assumes that the durability of ethnic boundaries is a function of the assimilation measures that sociologists commonly employ. But this literature fails to account adequately for the role of immigration patterns in explaining the durability and nature of ethnic boundaries. Using 123 in-depth interviews with later-generation Mexican Americans, this article shows that Mexican immigrant replenishment shapes ethnic boundaries and ethnic identity formation. The sizable immigrant population sharpens intergroup boundaries through the indirect effects of nativism and by contributing to the continuing significance of race in the lives of later-generation Mexican Americans. The presence of a large immigrant population also creates intragroup boundaries that run through the Mexican-origin population and that are animated by expectations about ethnic authenticity. The article illustrates the importance of immigrant replenishment to processes of assimilation and ethnic identity formation.