Twelve esters (C1-C6) of erythro- and threo-2-bromo-3-fluorosuccinic acid and related compounds were tested for antifungal activity against Candida albicans, Aspergillus niger, Mucor mucedo, and Trichophyton mentagrophytes at pH 5.7 and 7.0 in the absence and presence of 10% beef serum in Sabouraud dextrose agar. At pH 7.0 in the presence of 10% beef serum, no consistent pattern in the fungitoxicity of the erythro- and threo-2-bromo-3-fluorosuccinate esters was seen. Increasing the length of the ester function affects fungitoxicity as follows: C2 greater than C1 greater than C3 greater than C4 greater than C5 greater than C6. The most fungitoxic compound in this study was threo-ethyl 2-bromo-3-fluorosuccinate (C. albicans, 14 mug/ml; A. niger, 30 mug/ml; M. mucedo, 9 mug/ml; T. mentagrophytes, 5 mug/ml). Due to the ease of dehydrohalogenation, the fungitoxicity of 2-bromo-3-fluorosuccinic acid esters may be the result of a mixture composed of the parent compound, the bromo- and fluorofumaric acid esters, and HF and HBr of which part may be formed extracellularly and part within the cell.