Classical assimilation theory postulates that over time, members of immigrant groups will develop more primary ties with native members of the host society. However, lack of data has led most research to rely on the study of either spatial mobility or other secondary variables as proxies of primary ties. Using data from the Multi-City Study of Urban Inequality, this research examines primary relations, comparing the number of cross-ethnic strong ties among foreign- and native-born generations of persons of Mexican origin in Los Angeles County. The findings indicate that the native-born are substantially more likely to report cross-ethnic ties than immigrants. Spatial variables only partially explain the effect of primary structural assimilation, implying that both primary group and spatial dynamics play important roles in structural incorporation. (Contains 3 tables.)