Heronamide C, a polyketide macrolactam antifungal agent synthesized by marine Actinomycetes, exhibits selective toxicity by inhibiting growth and promoting abnormal cell wall material accumulation in wild-type fission yeast cells (MIC = 0.13-0.28 μM), without affecting Gram-positive bacteria or human cancer cell lines, HeLa and MDA-MB-231. At 20 μM, it triggers reversible morphological alterations, notably the development of large intracellular structures, in HeLa cells.