With the application of plate count methods and of the KOH-flexirubin test, bacteria belonging to the Cytophaga group were proved to occur regularly in samples from biological sewage treatment facilities. Generally, the percentage of Cytophaga colonies of the total heterotrophic colonies was lowest in the inflow sewage water as compared with the values found in activated sludge, trickling filter, and effluent samples. During an observation period of 16 months, the highest percentages of cytophagas were found in winter samples from activated sludge and trickling filters. Furthermore, cytophagas were shown to have high percentages of the bacteria lytic to polymeric substrates such as cellulose, chitin, dextran, pectin, xylan, and gelatin. Thus, it is suggested that cytophagas may contribute to sewage purification, especially at cold temperatures and by polymer breakdown. Cytophaga strains isolated were shown to have gliding motility, flexirubin pigmentation, and a low guanine plus cytosine base ratio in common. The strains were roughly subdivided into a spreading, a nonspreading, and a cellulolytic group.