This study attempts to measure the degree of assimilation exhibited by various immigrant groups, as indicated by their residential patterns. Ecological models of assimilation hold that immigrants are highly segregated from the majority population upon arrival, but that segregation declines with time in a process of residential assimilation. The general residential segregation of 13 ethnic groups, which vary in terms of immigrant proportion, was analyzed in Los Angeles (California) and San Diego (California). A more detailed analysis of ethnic segregation by year of immigration and citizenship status was performed for Los Angeles. Both analyses used metropolitan census tract data from the 1980 census. Results partially support the general models of residential assimilation. However, there was not a strict association between recency of immigration and the degree of segregation exhibited by the group. Immigrant assimilation occurs within an ethnic context. Further immigration to the United States is not likely to create immigrant ghettos or a residentially distinct underclass, because group ethnic identity appears to far outweigh immigrant status in determining segregation patterns. Statistical data are included on four tables and three graphs. A list of 56 references and a 25-item bibliography of related titles on the impact of immigration in California are also included. (FMW)