The in vitro activities of doripenem, imipenem, levofloxacin, piperacillin, ceftazidime, aztreonam, tobramycin, and cefepime were determined for 160 isolates of Pseudomonas aeruginosa (82 from cystic fibrosis [CF] patients) and 34 isolates of Burkholderia cepacia. Doripenem MIC90s were lower than those of all other comparative agents against all isolates combined and against all P. aeruginosa isolates. Doripenem was as active as levofloxacin and 2- to 32-fold more active than the other comparative agents against B. cepacia.