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 (Fig. 1). AA strongly inhibited the growth of Chinese cabbage seedlings at concentrations higher than 2.0 ppm in a germination and subsequent growth assay using Petri dishes, with a prominent swelling action on the base of the hypocotyl like that induced by high concentrations of auxin; its concentration-based inhibitory activity was about one tenth that of 2,4-dichlorophenoxyacetic acid. AA also showed high activity in a rice lamina inclination test for auxins and brassinosteroids at concentrations higher than 5.0 ppm. The structure of acremoauxin A, a novel and potent fungal auxin, was thus elucidated.