Three mutants of Emmonsia crescens with altered morphology and pathogenicity for mice are described. The mutant M-1 is avirulent and does not form adiaspores of normal morphology on agar medium at 37 degree C. The mutants M-2 and M-12 have a considerably decreased virulence. The pathogenicity of E. crescens depends on the ability to differentiate adiaspores of perfect morphology. The loss of virulence and the decrease of virulence of mutants seem to be caused by the pleiotrophic effect of mutation of the genetic base, which regulates the differentiation of adiaspores. The conidia of the wild strain of E. crescens killed by UV-irradiation do not induce the formation of granulomas with adiaspores after i.p. inoculation into mice.