The first section of this review covers the important characteristics of the genera Cytophaga and Sporocytophaga. The topics discussed include vegetative cell structure, the spreading habit, and degradation of macromolecules.A historical account of these two genera follows, together with a discussion on the definition of, and species differentiation within the genus Cytophaga, and on the taxonomy of Sporocytophaga.The third section deals with the relationships of the cytophagas with the flavobacteria and includes a brief history of Flavobacterium, reassignation of some species to Cytophaga, differentiation from Cytophaga, and a discussion on the definition of the genus Flavobacterinm.This is followed by a section dealing with the relationship of Cytophaga with the flexibacteria, starting with an introduction to the diversity of flexing organisms and taxonomic developments, and proceeding with the differentiation within the family Cytophagaceae, and species differentiation in Flexibacter.The concluding section includes a proposed redefinition of Cytophaga, a proposal regarding species conservation in this genus, and discussions on the relationship between the cytophagas and the myxobacteria and on the significance of cytophagas in the environment.The characteristics of all described species of Cytophaga, Flexibacter, and relevant flavobacteria are tabulated and a bibliography is presented.