Four 5-amino-4-alkylthio-pyrazoles were synthesized and their antifungal activity was evaluated in vitro in Trichophyton mentagrophytes, Microsporum cookei and Candida albicans. The compounds slightly influenced the growth kinetics of the yeast, but at concentrations ranging from 20 to 40 micrograms/ml completely prevented the mycelial growth of the two dermatophytes cultivated on Sabouraud's agar medium. An electron microscopic study, undertaken by using the most active compound, showed that in C. albicans mitochondria were the only cell targets affected whereas in the dermatophytes cell wall, plasmalemma and the main cytoplasmic organelles were damaged in various degrees. Since the most remarkable alterations were connected with membrane abnormalities, the cytological changes observed were tentatively interpreted as a consequence of the compound intrusion into the lipid bilayer of the membranes, since the drug is lipophilic in nature.