The study examines the effects of time in the United States and Indian ethnicity on prevalence of 12 DSM-III-R psychiatric disorders among Mexican Americans in California. In Fresno County, primarily an agricultural area, 3012 participants of Mexican origin (18 to 59 years) were selected under a cluster sampling design and interviewed using a version of the World Health Organization's Composite International Diagnostic Instrument (WHO-CIDI). Lifetime prevalence of any psychiatric disorder was 46.4% for Indians and 32.9% for non-Indians. Alcohol dependence was the most prevalent disorder (Indians = 17.4%, non-Indians = 10.7%). Indians had significantly higher risk of affective disorders (adjusted OR = 2.9) and drug abuse/dependence (adjusted OR = 2.6) compared with non-Indians. Time in the United States was associated with higher risk of lifetime affective disorders and drug abuse/dependence. This effect was more pronounced among Indians. Mexican immigrants are ethnically heterogenous and Indians appear to be more vulnerable to negative effects of exposure to U.S. society.