The correct identification of isolated E. coli strains is usually without problems in the bacteriological routine diagnostic. We recently isolated H2S-producing strains from urine samples of two patients. 4 strains isolated at different times from one patient were serotyped, as O21:H25 and 2 strains from another patient belonged to the serotype O86:H-. The isolates biochemically differed both in the reaction of ornithine decarboxylase and in the fermentation of sucrose (Table 1). Differences were ascertained in the agar diffusion test in the sensitivity against discs of ampicillin, carbenicillin, chloramphenicol, tetracycline and trimethoprim-sulfamethoxazole (Table 2). The present knowledge of the mechanisms for producing H2S by E. coli is discussed.