This study focuses on the behavior patterns of migrants and immigrants in the Mexican enclaves of Chester county, Pennsylvania, as they relate to the attempts of the Census Bureau to enumerate them in the decennial census. These enclaves are formed by migrants from the same areas in Mexico who over the years recruit their relatives, friends and neighbors to recreate in the U.S. a community similar to what they had in Mexico. Although the earlier workers tend to settle down in the new community, many of them will still travel to visit family back in Mexico. All continue to receive and house new migrant workers, arrimados, from Mexico, so that the bulk of the present enclave population consists of young migrants, predominantly male, largely undocumented, and monolingual in Spanish. These migrants are scattered throughout the area in hidden employer-provided housing, doubled- up quarters of previous immigrants, or crammed into migrant-shared rented houses or apartments. The precarious legal and employment status of the migrants, coupled with 1) the employers' concern over being penalized for hiring "illegal aliens" and consequently losing them to immigration authorities at peak work times, 2) substandard working conditions for which the labor unions could call them to task, and 3) the landlords' concerns over violating housing codes by providing substandard housing or overcrowding, are formidable barriers to accurate coverage. On the positive side, most of these new migrants are related to the older area residents and are recruited and taken in by them, and thus become part of the local social networks. They make use of community services, share resources and information, and generally can be reached through other members of the community.