This brief report presents estimates of the number of unauthorized immigrants in the United States in mid-2001 for three separate groups: the total unauthorized population, the Mexican unauthorized population, and the non-Mexican Central American unauthorized population. The approach to estimation used is one set forth recently by Bean, et al (2001) that extends and amplifies work originally begun as part of the Mexico/U.S. Binational Migration Study (1997; Bean, et al. 1998). The specific features of the approach are described in detail in Bean, et al (2001). Basically, the method involves subtracting estimates of the numbers of persons residing in the country legally from the numbers of foreign born persons in official government surveys (which are known to contain both legal and unauthorized persons), and then adjusting for extra undercount of such persons in the surveys. The resulting figures give estimates of various unauthorized populations in the country.