During January-June 2010, three Enterobacteriaceae isolates carrying a newly described resistance mechanism, the New Delhi metallo-beta-lactamase (NDM-1), were identified from three U.S. states at the CDC antimicrobial susceptibility laboratory. This is the first report of NDM-1 in the United States, and the first report of metallo-beta-lactamase carriage among Enterobacteriaceae in the United States. These isolates, which include an Escherichia coli, Klebsiella pneumoniae, and Enterobacter cloacae, carry blaNDM-1, which confers resistance to all beta-lactam agents except aztreonam (a monobactam antimicrobial); all three isolates were aztreonam resistant, presumably by a different mechanism. In the United Kingdom, where these organisms are increasingly common, carriage of Enterobacteriaceae containing blaNDM-1 has been closely linked to receipt of medical care in India and Pakistan. All three U.S. isolates were from patients who received recent medical care in India.