The isolation of sirodesmin H, a fungal toxin from Phoma lingam, is described. This is the first example of a monosulfur bridged dioxopiperazine. Sirodesmin PL (1) is the major non-specific phytotoxin produced by P. lingam, and the epipolythiodioxopiperazine group present in sirodesmin PL is common to a wide variety of fungal metabolites with di-, tri-, and tetrasulfur bridges, which are toxic to diverse organisms including mammals. As part of a plant breeding programme selecting for resistance to P. lingam, chromatography of culture medium extracts afforded sirodesmin PL and other components showing phytotoxic activity. Herein, we report the isolation and identification of the first naturally occurring toxin containing the epimonothiodioxopiperazine group, named sirodesmin H (2). Sirodesmin H is about ten times less phytotoxic than sirodesmin PL. The biosynthesis of the polysulfur bridge has been suggested to proceed by stepwise addition of sulfur, and the isolation of sirodesmin H, the first compound with a monosulfur bridge, supports this hypothesis.