More than 100 strains of C. genitalium, probably responsible for coryneform urethritis and other infections, and 600 commensals of the male and female [human] urogenital tracts were studied and grouped into 5 pathogenic types numbered I-V and 6 saprophytic types designated C-1 to C-6 on the basis of 8 biological reactions. This preliminary classification was based on differences in requirements for O2, on the fermentation of fructose, dextrose, sucrose and starch together with the production of the enzymes gelatinase, lipase and urease. One criterion differentiated the pathogens from the commensals: all pathogens were nonfructose fermenters whereas every commensal fermented this sugar.