The studies involved 37 domestic strains of the Trichophyton verrucosum species, which plays a dominating role in the etiology of skin mycosis in cattle. The pathogenesis of these strains was determined in guinea-pigs and calves, as well as their immunogenicity and antigenic structure. Differences in pathogenic properties of the individual strains were found both for guinea-pigs and calves. By the method of successive reinfections it was shown that trichophytosis cannot be evoked in guinea-pigs for the second time, and no relationship was found between the presence of antibodies in the serum of the animals and their immunity. Differences of antibodies were detected. However, no correlation between this feature and the virulence of the strain was found. Differences in the activity of extracts according to Fuller from the individual strains were observed in relation to positive rabbit serum against T. verrucosum, strain No 43. The comparison of the antigenic structure of the strains studied showed that they are identical. The methods used made it possible to distinguish 6 antigen fractions, of which fraction III was found in 78% of strains. The author did not succeed in identifying a specific antigen common for all T. verrucosum strains studied. From the analysis of the properties studied, only one strain (No 8) was isolated of 37 T. verrucosum strains, which is useful in every respect for production of live vaccine against trichophytosis.