Amidenin, a new plant growth-regulating substance, was isolated from the fermentation broth of an Amycolatopsis sp. The active principle was extracted with ethyl acetate and purified by silica gel column chromatography. The molecular formula of this substance was established as C10H17NO, and the structure was determined to be 2E,4Z-decadienamide by spectroscopic analyses. This substance showed growth-promoting activity with a 0.6×10^-5 -1.8×10^-5 M treatment to rice seedlings, and inhibitory activity with a 6.0×10^-5 M treatment. We have been screening new active substances from metabolites of actinomycete and other bacteria for the root elongation of rice seed, and have previously reported a new plant growth-regulating substance designated as thienodolin. 1•2) We also found another new active substance and named this amidenin (1). The taxonomy of the producing organism, together with the fermentation, purification, physico-chemical properties, structural elucidation and biological properties of 1 are described in this report.