Freshly expectorated sputum from a 52-year-old man with extensive pulmonary blastomycosis was examined by light and electron microscopy. Wet preparations of the sputum, stained with 1% aqueous cresyl violet, revealed numerous thick-walled, spherical blastospores and occasional budding forms. Ultrastructurally, the blastospores revealed a double-layered cell wall, the peripheral portion of which consisted of a thick, densely staining lamellar material of variable thickness, suggestive of the glycocalyx of bacterial cells. Mitochrondria and other organelles were easily recognized, but distinct nuclei were not observed. Retraction of the cell membrane and cytoplasmic mass from the cell wall was a common finding. Results of this case study suggest that electron microscopy of sputum may serve as an important adjunct to conventional staining techniques for the presumptive diagnosis of blastomycosis. Similar studies of other fungi in respiratory tract secretions may be helpful in providing more precise morphologic identification.