Since 1997, the Meropenem Yearly Susceptibility Test Information Collection (MYSTIC) Program has monitored the antimicrobial activity of broad-spectrum agents against pathogens from hospitalized patients. In the United States, 2894 isolates were submitted in 2007 from 15 sites, including 1392 Enterobacteriaceae, 643 nonfermentative Gram-negative bacilli, and 829 Gram-positive cocci. All isolates were tested by broth microdilution methods. Meropenem (MIC(90) range, 0.12-2 microg/mL) exhibited the lowest resistance rates (1.9-2.4%) against Enterobacteriaceae, and fluoroquinolones had the highest rates of resistance (17.3-18.3%). KPC carbapenemases, usually found in Klebsiella pneumoniae, were also detected in Citrobacter freundii, Enterobacter spp., and Escherichia coli. Confirmed extended-spectrum beta-lactamase-producing isolate rates for E. coli, Klebsiella spp., and Proteus mirabilis isolates were 6.0%, 12.0%, and 0.0%, respectively. Meropenem remained active against Gram-positive pathogens such as staphylococci (methicillin-susceptible; MIC(90), 0.12-0.25 microg/mL), Streptococcus pneumoniae (MIC(90), 0.5 microg/mL), and beta-hemolytic and viridans group streptococci (MIC(90) range, 0.06-0.25 microg/mL). These US MYSTIC Program results demonstrate the continued emergence of novel beta-lactamases and multidrug-resistant bacterial phenotypes necessitating monitoring of carbapenem activities against Enterobacteriaceae species as well as nonfermentative bacilli.