Sialic acids were characterized on the cell surface of conidia and hyphae of Fonsecaea pedrosoi, one of the agents of chromoblastomycosis. Neuraminidase-treated conidia had a reduced negative electrophoretic mobility and, in comparison with untreated cells, bound fewer particles of colloidal iron hydroxide and of cationized ferritin. Sialic acid residues in conidia are linked to galactopyranosyl units as indicated by the increased reactivity of neuraminidase-treated cells with peanut agglutinin. N-acetylneuraminic acid was the only derivative found in the mycelium whereas conidia contained both N-glycolyl- and N-acetylneuraminic acids.