The tube dilution method was used to test 33 strains of Pseudomonas pseudomallei for in vitro sensitivity to trimethoprim (TMP), sulfamethoxazole (SMZ), and a combination of the drugs maintaining a ratio of 20 parts SMZ to one part TMP. Results disclosed no activity by TMP alone except at extremely high levels. Twelve of 33 strains were sensitive to SMZ, and 75% of the strains were inhibited by the combination at therapeutically achievable levels. In no instance did TMP fail to potentiate the action of SMZ against the organism. In vitro susceptibility of the organism indicates that a combination of TMP and SMZ could be useful in the treatment of melioidosis.