474 Klebsiella pneumoniae and K. oxytoca strains isolated from different sources (human clinical material, feces of healthy subjects, sewage) were investigated for phenotypic properties. Characteristics analyzed were cultural activities, antimicrobial susceptibilities and capsule types. Comparison of both species revealed differences in adonitol fermentation and resistance to tetracycline, nalidixic and pipemidic acid. Capsule types 2, 7 and 33 were frequently found in K. pneumoniae, but not in K. oxytoca. On the other hand, K 66 was common in K. oxytoca, but not in K. pneumoniae. With regard to the source of isolation, clinical strains of both species proved to be more resistant to mezlocillin, azlocillin and cephalothin than fecal and sewage strains. Similarly, resistances of K. pneumoniae to cotrimoxazole, nalidixic and pipemidic acid were most frequent in clinical strains. Multiple drug resistances were found most often in clinical isolates. Biochemically, different frequencies of positive reactions for urease, lysine decarboxylase activity and acetoin production were found between the groups. Capsule typing demonstrated K2 and K7 in K. pneumoniae and K55 and K66 in K. oxytoca to be more common in clinical and fecal isolates than in sewage strains. While cultural characteristics did not allow discrimination of strains from different sources, capsule typing indicated clinical isolates to be more phenotypically related to strains from feces than to sewage isolates.