During 2007-2008, 1,036 gram-negative bacilli were isolated from patients with complicated intra-abdominal infections in the United States. Against members of the family Enterobacteriaceae, the most active agents in vitro were ertapenem, imipenem, and amikacin, while the least active agent was ampicillin-sulbactam. Ertapenem and imipenem were active against all extended-spectrum-beta-lactamase (ESBL)-positive Escherichia coli. Antimicrobial resistance in gram-negative bacilli isolated from patients with complicated intra-abdominal infections in the United States continues to increase.