Siccanin is an unusual fused phenolic pentacycle first isolated from Helminthosporium siccans and reported in 1962 as a potent antifungal agent. Siccanin inhibits succinate dehydrogenase in the terminal electron transport system. More recent studies note the proximity of the siccanin binding site to the quinone-binding site of the enzyme. Species-selective inhibition by siccanin is unique among succinate dehydrogenase inhibitors and offers a lead for new chemotherapeutics.