Twenty-three 4-substituted crotonic acid esters were tested for antifungal activity against Candida albicans, Aspergillus niger, Mucor mucedo and Trichophyton mentagrophytes. For the analogues of the methyl ester containing substituents in the 4 position, the following order of fungitoxicity was observed: I > Br > Cl > CH3S > CH3O > F = H. Of the homologues of the esters of the 4-iodo and 4-bromo compounds which included methyl, ehtyl, n-propyl, n-butyl, n-pentyl and n-hexyl, ethyl 4-iodocrotonate was most toxic to the 4 fungi at pH 7.0 in the presence of 10% beef serum (C. albicans, 18 .mu.g/ml, A. niger, 40 .mu.g/ml, M. mucedo, 5 .mu.g/ml, T. mentagrophytes, 4 .mu.g/ml). The mechanism of fungitoxicity may be due, in part, to a nucleophilic reaction involving SH-containing compounds. This is based on the correlation of fungitoxicity with the order of leaving groups in the nucleophilic reaction and the protection against the toxicity of the test compounds to the fungi by cysteine and glutathione.