A study examined various characteristics of the employment of illegal aliens, including wages, job training, job mobility, workplace conditions, and employer characteristics. The study was largely based on data transcribed from a sample of Immigration and Naturalization Service apprehension reports on illegal aliens in the Chicago (Illinois) metropolitan area and an extensive interview of businesses in the Chicago metropolitan area (half of which were identified by illegal aliens, with the other half being randomly selected). Data on the labor market behavior of nearly 300 male illegal aliens apprehended in the Chicago metropolitan area indicated that the average alien was 30.6 years old, had been in the United States an average of 3.4 years, and received $4.52 per hour (in 1983) as opposed to $4.42 received by Mexican nationals and $4.73 received by other nationals. Only 16 percent (mostly in restaurant and service jobs) reported wages below the federal minimum level. Businesses employing illegal aliens were more likely to exhibit seasonal employment patterns and provided significantly less on-the-job training than did their counterparts that did not employ illegal aliens. The illegal aliens studied tended to exhibit considerable job mobility, with Mexican illegal aliens exhibiting stronger ethnic enclave patterns of employment than did the other ethnic groups. (Twenty-five tables are included.) (MN)