On 9 March 1996, a 57-year-old Japanese drunken male drown in a public bath in Tokyo. He was transferred to a emergency hospital and recovered. After his discharge on 11 March by walking, he became febrile at night. Next day, because of high fever and dyspnea, he came to the medical attention, and was immediately hospitalized under the diagnosis of acute pneumonia. Although bacteriological, serological examinations and chemotherapy for suspected Legionella pneumonia, definite diagnosis was not obtained and the patient died on 6 April. Culture of the autopsied lung tissue yielded colonies of Legionella pneumophila serogroup (SG) 6, and reexamined serum antibody titer against. L. pneumophila serogroup 6 was 1: 1024 by microplate agglutination test. Examinations for legionellae and their host amobae in the water of 22 bath tubs of 6 public bath facilities located in the area including the facility concerned were carried out on 22 April without notification in advance. Free residual chlorine concentrations of the 22 bath water were from 0.1 to more than 5 mg/L, and water from 2 bath tubs (0.1%) of low chlorine level were legionellae-positive. Host amoebae for legionellae were detected from 10 bath tubs of 5 facilities. Though Naegleria was detected, the bath water where the patient drowned was negative for viable legionellae by repeated trials of culture, 3 times intraperitonal passages of guinea pigs, and coculture with amoebae. The 16S rRNA gene specific for legionellae was detected from the bath water by nested PCR method using primers, 225A-854B and 448A-854B. After filtration of 10 ml bath water, the membrane filter was stained by indirect fluorescent antibody (IFA) method. Rodshaped organisms trapped on the membrane filter were IFA-positive against L. pneumophila SG 6, same with the isolates from lug tissue, and their presumptive number in bath water was estimated as 102-103/ml. Based on the results of nested PCR and IFA staining of rod-shaped bacteria trapped on the membrane filter, the bath water was regarded as contained with viable legionellae due to unknown reason and could be the source of infection when the patient was drowned.