A new unusual phytotoxic bis-amino acid N-glucoside, named ascaulitoxin, was isolated from the culture filtrate of Ascochyta caulina, the causal agent of leaf and stem necrosis of Chenopodium album, a promising mycoherbicide for the biological control of this common noxious weed. Ascaulitoxin, characterized by extensive use of NMR techniques and chemical methods as N2-(2,4,7-triamino-5-hydroxy)-octanedioyl-β-D-glucopyranoside, showed phytotoxic activity against host and non-host plants.