This study used structural equation modeling to test a model of ethnic identity development among 513 Mexican-origin adolescents living in the United States. The model examined the influence of ecological factors, familial ethnic socialization, and autonomy on adolescents' ethnic identity achievement. Findings indicated that lower percentages of Mexican-origin individuals attending adolescents' schools and fewer members of adolescents' immediate family born in the United States were each associated with greater familial ethnic socialization; furthermore, familial ethnic socialization was positively related to ethnic identity achievement. These findings suggest that ecological factors indirectly influence ethnic identity achievement through their influence on familial ethnic socialization.