During a search for elicitors and phytotoxins produced by virulent isolates of the phytopathogenic fungus Leptosphaeria maculans (Desm.) Ces. et de Not. [asexual stage Phoma lingam (Tode ex Fr.) Desm.], the selective phytotoxin maculansin A was isolated and its structure determined by analysis of spectroscopic data and chemical degradation. Maculansin A, a unique derivative of mannitol containing the unusual chromophore 2-isocyano-3-methyl-2-butenoyl, was isolated from potato dextrose cultures of L. maculans virulent on canola (Brassica napus L. cv. Westar). Surprisingly, maculansin A was more toxic to resistant plants (B. juncea L. cv. Cutlass, brown mustard) than to susceptible plants (canola). Maculansin A, however, did not elicit the production of phytoalexins either in resistant or susceptible plants. In addition, other maculansin type structures and the metabolite 2,4-dihydroxy-3,6-dimethylbenzaldehyde were isolated and the latter was found to be a strong inhibitor of root growth of both brown mustard and canola. Considering that L. maculans seems to be expanding its host range to infect brown mustard as well, maculansins could assist in chemotaxonomic studies to group the diverse isolates.