Mariannaeapyrone ((E)-2-(1,3,5,7-tetramethyl-5-nonenyl)-3,5-dimethyl-6-hydroxy-4H-pyran-4-one) is a new fungal metabolite isolated from fermentations of the common mycophilic deuteromycete Mariannaea elegans. The chemical structure of the 4-pyrone was determined by spectroscopic techniques. Mariannaeapyrone is a selective inhibitor of the thromboxane A2 induced aggregation of human platelets, whereas only weak cytotoxic and antimicrobial effects could be observed.