Many western liberal democracies have witnessed increased bias against immigrants and opposition to multiculturalism. Prior research suggests that ethno-linguistic differences between immigrant and native populations are a key cause of that bias due to the perception of cultural threat. Linguistic assimilation has been proposed as the key mechanism to reduce bias and mitigate conflict between natives and immigrants. Using a large-scale field experiment in Germany—a country with a high influx of immigrants and refugees—we show that linguistic assimilation does not reduce bias. We find that Muslim immigrants are no less likely to be discriminated against if they appear to be linguistically assimilated. However, we also find that ethno-linguistic differences do not cause bias among German natives, suggesting that Germany may have already reached a relatively high level of tolerance to multiculturalism.