Four studies were conducted to test whether assimilation ideology affects majority group members’ attitudes towards ethnic minorities. Assimilation affirms and justifies the identity of majority groups and highly-identified group members are motivated to think and behave in the ingroup’s best interest. Therefore, it was expected that assimilation would make higher identifiers more negative. The first two studies focused on the individual endorsement of assimilation and the other two used an experimental design to encourage participants to think in terms of assimilation. Results in all four studies show that assimilation is related to more negative outgroup attitudes, especially for higher majority group identifiers. The findings are discussed in relation to attempts to rethink and rehabilitate assimilation theory, and to other ideologies for dealing with cultural diversity.