In this paper's model, undocumented workers are endogenously sorted into secondary labor markets. When further illegal immigration occurs, some new migrants follow their fellows into already migrant-dominated jobs, lowering migrant wages and raising real incomes of host-country labor and capital. Some submarkets switch from employing legal workers to employing migrants, lowering demand for and wages of legal workers. Undocumented immigration is Pareto-improving when enforcement reserves primary-sector jobs for legal workers. Pareto-dominant policies target the number of migrant-dominated submarkets, not the number of migrants. This appears consistent with U.S. enforcement practices. The effects of deportations, employer sanctions, and amnesties are explored.