The activity of carbenicillin (CB) on 20 strains of Pseudomonas pseudomallei was determined by the agar-diffusion and tube-dilution methods. Under the conditions of the tube-dilution method oxygen diffusion limited the growth of this obligate aerobe and KNO3 (0.1%) was added to the assay medium as an alternate electron acceptor. The range of minimal inhibitory concentrations (MIC) for the strains studied was 50 μg/ml, 100 μg/ml, and 200 μg/ml. Thus, 17 of the 20 strains had MIC's below the blood level of CB achievable in vivo (in excess of 100 μg/ml). Activity of CB declined in the presence of heavy inocula and early logarithmic phase cells were more sensitive to CB than stationary phase cells. Variation in incubation temperature or electron acceptor availability caused changes in the MIC.Survival curves of P. pseudomallei in the presence of CB (at the minimal bactericidal concentration (MBC)) indicated a loss of colony-forming units at a rate of 1 log per 1.75 h. The increase in OD of P. pseudomallei cultures in the presence of CB was found to be due to filament formation.