The antifungal activity of 6-acetamido-2-alkylthiobenzothiazoles (altogether 13 derivatives) against Aspergillus niger, Candida guilliermondii and against the yeast-like and mycelial forms of Candida albicans was determined. No efficacy of more significance was found against A. niger. 6-Acetamido-2-n-butylthiobenzothiazole, which with the ED50 = 23 micrograms/ml ranks among the agents of medium efficacy, was most effective against C. guilliermondii. Good efficacy was found in 6-acetamido-2-n-pentylthio- and 6-acetamido-2-n-hexylthiobenzothiazole against the mycelial form of C. albicans IC95(M) = 40 mumol/dm3 = 12 micrograms/ml), which is minimally five times higher efficacy than in the case of 2-mercaptobenzothiazole.