We have recently isolated gibberellins A4 and A9 from a new gibberellin-producing fungus, Phaeosphaeria sp., and an abscisic acid-related compound, (2Z,4E)-( + )-4'-hydroxy-γ-ionylideneacetic acid, from Stemphylium sp. The continuing search for new plant-growth regulators from fungal metabolites revealed the presence of a potent plant-growth inhibitor with auxin activity in a culture medium of Acremonium roseum I 4267. The structure of this new plant-growth inhibitor, named acremoauxin A (AA), isolated from the culture filtrate was elucidated as I. AA strongly inhibited the growth of Chinese cabbage seedlings at concentrations higher than 2.0 ppm in a germination and subsequent growth assay, showing a prominent swelling action on the base of the hypocotyl like that induced by high concentrations of auxin, with its concentration-based inhibitory activity being about one-tenth that of 2,4-dichlorophenoxyacetic acid. It also showed high activity in a rice lamina inclination test for auxins and brassinosteroids at concentrations higher than 5.0 ppm. Acremonium roseum I 4267 was cultured in shaken Sakaguchi flasks containing a 2% malt extract-2% dextrose-0.1% peptone medium for 8 days at 25°C. The culture filtrate was partitioned against EtOAc, and the evaporated EtOAc extract was fractionated on a silica gel column to obtain AA as colorless needles (mp 142-144°C, [α]D +53.6°(MeOH)). Structural elucidation using FDMS, HR-EIMS, IR, UV, 1H-NMR, alkaline hydrolysis (yielding (2S)-2-(3-indolyl)propionic acid as an acidic component), and comparison with authentic specimens and synthetic model compounds confirmed that AA consists of (2S)-2-(3-indolyl)propionic acid esterified to the 1-hydroxyl group of D-arabitol.