This paper examines the demographic and socioeconomic patterns of 7 Mexican-origin and U.S.-born subgroups living in 13 midwestern states in 1990. Mexican-born immigrants are categorized into five subgroups based on the period of U.S. entry: pre-1965, 1965-74, 1975-81, 1982-86, and 1987-90. U.S.-born Mexican Americans (as well as those born abroad to U.S. citizens) are classified into two subgroups: born in the Midwest and born elsewhere. The final analysis compares Mexican immigrants in the Midwest who came to the United States between 1980 and 1990 with those living in other regions of the country. Data are from the 1990 Public Use Microdata Samples of the U.S. Bureau of the Census. Analyses examine population size, geographic distribution among the 12 states, rural versus urban residence, age and sex structure, percent U.S. citizenship, percent speaking English well or very well, percent high school graduates, unemployment, occupational level, income, and percent of families in poverty. Among the findings are: (1) most Mexican immigrants in the Midwest lived in Illinois (particularly Chicago) or in other metropolitan areas; (2) immigrant groups showed a straight-line increase in assimilation (U.S. citizenship and English language proficiency) with length of time in the United States; (3) educational attainment varied widely, but the least educated groups were immigrants arriving since 1975; and (4) the two earliest Mexican-origin immigrant groups were better off socioeconomically than the other groups, including U.S.-born Mexican Americans. Contains 26 references and 2 large data tables. (SV)