The dermatophyte Microsporum cookei Ajello was treated with nine new natural and synthetic 2,2':5',2"-terthiophenes to determine their possible antifungal activity. In the dark the thiophenes were inactive, while when photoactivated with UV-A they induced a remarkable reduction in the growth rate of the fungus. The only exception was (E)-N-(2-methylpropyl)-3-(2,2':5',2"-terthien-5-yl)-propenamide , which was not fungistatic even at the highest dose tested (24 microM). The more active compounds were 3'-methoxy-2,2':5',2"-terthiophene and 3'-methylthio-2,2':5',2"-terthiophene, whose activity seems to be related to the presence of a substituent in the 3' position of the central ring of thiophenes. Transmission electron microscopic observations demonstrated the photoactive nature of the synthetic molecules to be similar to that of alpha-terthienyl, a natural thiophene present in some Asteraceae. The dark treatment caused only the accumulation of the compound in vacuoles, without other evident alterations. After UV-A irradiation the activated thiophene causes severe modifications to the endomembrane system, probably via oxygen-dependent mechanism.