Sporidesmin (C,,H,,,,,~ClN,O,S,) and sporidesmin-B (C,,H,,C1N3O,S,), toxic metabolites of Pithomyces C~CCY~UYU~, have been isolated and characterised. Each contains two methoxyl, two N-methyl, and hydroxyl groups. The reactivity of the sulphur in sporidesmin is similar to that in gliotoxin. On alkaline degradation ammonia, methylamine, and a red ketone were isolated from both sporidesmins. It is concluded that sporidesmin is a hydroxysporidesmin-B.