The pathogenicity of seven morphological mutants of Emmonsia crescens was tested by means of intraperitoneal inoculation in mice. All mutants caused adiaspiromycosis. Adiaspores were isolated from granulomas after 2 months and their diameters were determined. Adiaspores from granulomas caused by five mutants (M-5, M-6, M-8, M-9 and M-16) were significantly smaller than adiaspores from granulomas caused by the wild strain, from which the mutants were derived. Two mutants (M-6 and M-9) produced adiaspores of the smallest diameter (130.5 and 119.9 mum) with the lowest variance of values, differing thus most from the original wild strain with adiaspores of 230.4 mum in diameter. A positive correlation was found between the size of the adiaspore in vivo and growth rate of the mycelial stage of Emmonsia crescens in vitro. The mutation characterized by the decreased growth rate of the mycelial stage is phenotypically manifested in the adiasporic stage of the life cycle of Emmonsia crescens, i.e. by the smaller average size of adiaspores in granulomas.