A minor product of the bacterial plant pathogen 'Pseudomonas andropogonis' was isolated and purified, and identified as dihydrorhizobitoxine by mass and NMR spectroscopy by comparison with authentic material. The production of dihydrorhizobitoxine from a total of five rhizobitoxine-producing strains of P. ondropogonis was established by isolation and purification of the compound and comparison with authentic dihydrorhizobitoxine. In contrast, three strains of P. andropogonis that did not produce rhizobitoxine, also failed lo produce dihydrorhizobitoxine. When P. andropogonis was fed 4-'3C-asparfic acid, the dihydrorhizobitoxine had a four-fold enrichment at C-4, as has been found with rhizobitoxine, and shows the specific utilization of aspartic acid in the biosynthesis of dihydrorhizobitoxine. A _ l.8-fold enrichment at C-l of dihydrorhizobitoxine most likely occurred by mixing of carbons 1 and 4 of aspartic acid through the action offumarase. Implications for the biosynthesis of both rhizobitoxine and dihydrorhizobitoxine are discussed.