The antifungal agent rapamycin is highly effective in inhibiting growth of yeast and mold strains. This study demonstrates that in liquid medium, rapamycin is more active than its derivatives (prolylrapamycin, 32-desmethylrapamycin, 32-desmethoxyrapamycin) against Candida albicans, Saccharomyces cerevisiae, and Fusarium oxysporum. All the rapamycins were more active than amphotericin B. Although four other molds were not inhibited in liquid medium, they were very sensitive to rapamycin and its derivatives when tested on agar. The latter assay showed that rapamycin is the most active and 32-desmethylrapamycin is more active than prolylrapamycin and 32-desmethoxyrapamycin. The conclusion of this study is that rapamycin is the most active antifungal agent of the compounds examined. The unexpected finding of high activity of rapamycin and its derivatives against filamentous fungi when assayed by the agar diffusion assay suggests that rapamycin or a derivative may hold promise for chemotherapy against pathogenic molds as well as yeasts.