The morphological changes in the hyphae of Trichophyton mentagrophytes treated with Naftifine (0.01-0.5 micrograms/ml) were studied by light and electron microscopy. The most striking changes observed following treatment with this new antimycotic agent were bulb-shaped thickenings at the hyphal tips and dose-dependent, spherical, or drop-shaped depositions of varying size within the cells. The abnormal formations were not only visible in the cytoplasm (discrete or aggregated in vacuoles), but also in the region of the cell membrane, in all layers of the cell wall and on the cell surface. Their lipid nature can be deduced from several significant characteristics including osmiophily, the conchoidal fracture surface observed in freeze-fracture replicas and their extractability with acetone. This evidence suggests that the antimycotic action of Naftifine results from interaction of the agent with fungal lipid metabolism and possibly from alterations of the cell wall structure. The latter is also suggested by the changes observed in the hyphal tips.