Public opinion toward illegal migration to the United States varies considerably across different segments of the population, but little is known about why some individuals hold more liberal attitudes than others. Several hypotheses are scattered throughout the research literature, but they have not been brought together in one place and tested using a common data set. Nor have the limited tests been satisfactory from a methodological standpoint. Instead of using multiple regression, typically analysts have relied on cross-tabulations of the data. This paper tests five hypotheses about attitudes toward illegal immigration and undocumented migrants using public opinion data from southern California. Only weak support is found for a labor market competition hypothesis. There is firmer evidence for hypotheses relating to cultural affinity between respondents and undocumented migrants and to the role of education. Respondents' evaluations of tangible costs and benefits to themselves also influence their assessments of illegal immigration. Finally, the results of this analysis provide additional support for a symbolic politics model of opinion formation when the model is extended to the issue of undocumented migration to the United States.