One hundred and eight epidemiologically distinct strains of gentamicin-resistant Kiebsiella aerogenes were selected from 404 patient-isolates sent from 12 hospitals in six countries. Serotypes K2, K17 and K21 were commonest and together accounted for 33 3% of strains. Resistance to many antibiotics in addition to gentamicin was common; 99 strains were resistant to at least 10, and 31 to at least 15, of the 24 antimicrobial agents tested. All strains were resistant to carbenicillin and ampicillin, and more than 70% were resistant to streptomycin, neomycin, kanamycin, tobramycin, cephaloridine, cephalothin, sulphamethoxazole, tetracycline and chloramphenicol. Forty-four per cent were- resistant to trimethoprim. MIC determinations suggest that cefotaxime, cefuroxime, cefoxitin and amikacin, with resistance rates of 0, 1, 2 and 4% respectively, are the drugs of choice. Consideration should be given to defining antibiotic policies that reserve these agents for these exceptionally resistant opportunistic bacteria.