SUMMARY About go strains of gliding microbes (flexibacteria) have been considered. Data obtained by Lewin & Lounsbery (1969) and Mandel & Lewin (1969) have been used by Fager (1969) in a computer programme designed to indicate affinities and thereby possible relationships. Largely on the basis of Fager's analysis, a classification of these organisms is here proposed. Including a few species described elsewhere, 27 species and varieties are distinguished, most of them apparently new. A simplified diagnostic key is presented for their identification. They are assigned to six genera, three (Saprospira, Flexithrix and Herpetosiphon) are recognized primarily on mor- phological grounds ; three others (Cytophaga, Flexibacter and Microscilla), here somewhat re-defined, are based largely on other convenient charac- teristics.