A morphological mutant of Paracoccidioides brasiliensis strain IVIC Pb9 was isolated after treatment of the yeast-like (Y) form with nitrosoguanidine. Colonies of the mutant grown at room temperature did not show the whitish cotton-like morphology typical of the mycelial form of the parental strain. Y-cells were much smaller than those produced by the parent and grew forming chains of different sizes. The main chemical difference in the wall of the Y-form was the replacement of the alpha-I,3-glucan, typical of the parental strain, by an amorphous I,3-mannan in the mutant.