Meropenem is a new dehydropeptidase stable carbapenem which has an in vitro bacteriostatic activity superior to imipenem against Pseudomonas aeruginosa. Minimal inhibitory concentrations of meropenem (MEM) and imipenem (IPM) were determined on twelve strains: two were genetically characterized reference strains and ten isolated from patients. Killing curves were performed for MEM used at 1,2 and 4 fold the MIC and 32 mg/l; survivors were counted at baseline and after 5 and 24 hours of incubation by spiral plating. Five strains were susceptible to MEM and IPM; five were intermediate and two resistant to MEM; these seven strains were resistant to IPM. A weak bactericidal effect was obtained after 5 h at each concentration. Only one strain achieved a 3 log 10 reduction after 5 h at 2 x MIC. A regrowth of all the strains tested was observed after 24 h.