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The hyphal septa of 3 chromomycosis agents, Phialophora pedrosoi, Phialophora verrucosa, and Cladosporium carrionii were investigated ultrastructurally. Single pores were observed in the septa of all 3 species. Conspicuous bodies consistent in appearance with previous descriptions of Woronin bodies were also observed. These observations suggest an ascomycetous affinity for these imperfect dematiaceous fungi.

Chemical analyses demonstrated the similar compositions of hyphal walls isolated from the chromomycosis agents Phialophora verrucosa, P. pedrosoi and Cladosporium carrionii. Unfractionated walls of each species consisted of large amounts of glucose (17-31%) and protein components (29-42%) and smaller amounts of mannose (8-14%) and glucosamine (6-8%). Alkali-soluble wall fractions consisted predominantly of glucose and mannose, while alkali-insoluble wall fractions consisted predominately of glucosamine and protein components. Chitin was identified as a wall component by release of N-acetylglucosamine during enzymatic digestions of the walls with Streptomyces griseus chitinase. The presence of chitin in the walls and alkali-insoluble residues was confirmed by X-ray powder analysis. The suggestion is made that the hyphal forms of human pathogenic fungi have wall compositions intermediate between those of euascomycetes and hemiascomycetes.

The antigenic relationships among Phialophora verrucosa, Fonsecaea pedrosoi, and Cladosporium carrionii were studied by comparing water soluble antigens extracted from mycelia and extra-cellular antigens from culture filtrates. Immunodiffusion and immunoelectrophoresis tests detected common antigens among all 3 species, as well as antigens which appeared to be either genus or species-specific. Each test revealed more common antigens between P. verrucosa and C. carrionii than between either species and F. pedrosoi, indicating that P. verrucosa and C. carrionii are more closely related to one another than they are to F. pedrosoi. These findings provide justification for the retention of F. pedrosoi in the genus Fonsecaea.