Ashdown's selective-differential agar medium, with or without preenrichment in selective broth, was evaluated for the isolation of Pseudomonas pseudomallei from 1972 clinical specimens obtained from 643 subjects in Northeast Thailand; 226 patients proved to have meliodosis. The use of Ashdown's medium significantly increased the frequency of recovery of P. pseudomallei from sites or specimens with an extensive normal flora (throat, rectum, wounds and sputum) as compared to the recovery on blood and MacConkey agars (p less than 0.01). The isolation frequency from throat, rectal and wound swabs was further increased by the use of the broth pre-enrichment. The colonial morphology of P. pseudomallei on Ashdown's medium was sufficiently characteristic to allow presumptive identification. With the use of these selective media it was possible to culture P. pseudomallei from throat swabs taken from 87% of the patients from whom the organism could also be isolated from corresponding tracheal aspirates or sputum specimens. P. pseudomallei was isolated from rectal swabs taken from 51 patients, the first time that faecal excretion of the organism has been demonstrated in man. The diagnosis of melioidosis would not have been confirmed bacteriologically in eight patients (3.5%) without the use of the selective media. It is suggested that, in areas endemic for melioidosis, all sputum specimens should be cultured on selective media, such as Ashdown's. For the investigation of clinically suspected cases of melioidosis, and for follow-up during treatment of the disease, the use of broth pre-enrichment is recommended for specimens obtained from sites with an extensive normal flora.