From mycelial cultures of Oudemansiella mucida a crystalline optically active antibiotic, oudemansin (2), has been isolated; its structure is closely related to strobilurin A (1). The relative configuration of oudemansin have been determined by X-ray analysis. The antibiotic exhibits strong antifungal properties and inhibits respiration in fungi, cells of the ascitic form of EHRLICH carcinoma, and rat liver mitochondria. prove that it is different from mucidin. In the UV spectrum a maximum at 245 (loge 4.48) is observed, the IR spectrum in KBr disc is shown in Fig. 1. Structure ElucidationThe molecular formula C,-,H,209 points out a close relationship of oudemansin to strobilurin A (1). Like the latter oudemansin contains a methyl /3-rnethoxyacrylate system, which gives rise to signals in the "C nmr spectrum at S 51.6, 62.1, 113.1, 161.5, and 170.2 ppm". The 1H nmr indicates, Dedication: This paper is dedicated to Dr. FR]TZ LIPhtANN on occasion of his 80th birthday.