A total of 118 mycoplasma strains isolated from the conjunctivae and the respiratory‐ and genital tracts of 78 dogs were studied biochemically and serologically. Three biochemical tests (glucose fermentation, catabolism of arginine and phosphatase activity) were found to be very useful for a preliminary biochemical grouping prior to serological identification. Using these tests, the strains could be divided into six groups that did not show any antigenic overlapping. By serological examination using the indirect immunofluorescence technique and growth inhibition 35 isolates were identified asM. canis;21 were identified asM. edwardii;10 asM. cynos;10 as the feline speciesM. gateae;30 asM. spumans;2 asM. maculosum;2 as the bovine speciesM. bovigenitalium;and 5 asM. feliminutum, a species previously represented by a single strain isolated from a cat. The three remaining strains which were glucose positive and arginine and phosphatase negative were placed in a separate serogroup, group A, as they were serologically different from the established canine specie