A total of 195 patients admitted to a respiratory-surgical intensive care unit became colonized with species of Flavobacterium during a 70-month prospective study. By biochemical, cultural, and morphological criteria and a comparison of antibiotic susceptibilities, all patient isolates of Flavobacterium were apparently related. The origin of these organisms was sought. Flavobacterium were recovered from different water-associated areas of the hospital and from the hands of respiratory-surgical intensive care care unit staff. The organisms were also found in university dormitory sinks. The isolation of these organisms from tap water led to their recovery from reservoirs supplying drinking water to the city of Boston and surrounding communities. These organisms are resistant to chlorine concentrations found in municipal water. There was no proven case of pneumonia caused by Flavobacterium in 2,329 consecutive patients studied in our respiratory-surgical intensive care unit.