California's Proposition 187, directed primarily toward Mexican immigrants, deprives illegal immigrants of many benefits associated with U.S. citizenship and facilitates their deportation. The authors hypothesized that the respondents' opinions on this proposition would be determined by in-group-out-group bias (i.e., the tendency to evaluate the ethnic out-group more negatively than the ethnic in-group). In accord with that hypothesis, variations in respondent ethnicity (Studies 1 and 2) and in immigrant ethnicity (Study 3) were systematically related to the respondents' opinion on that issue. Moreover, the effect of in-group-out-group bias was independent of perceived reasoned economic and legal considerations that underlay the respondents' opinion.