AbstractSix 13C‐labelled isotopomers of mevalonolactone were synthesised and used in feeding experiments with the endophytic fungus Geniculosporium. The high incorporation rates of 13C‐label into a sesquiterpene that was found in headspace extracts of the fungus enabled unambiguous identification of this volatile as pogostol without the need for compound purification, simply by collecting the volatile fraction with a closed‐loop stripping apparatus followed by direct 13C NMR analysis (CLSA‐NMR). The feeding experiments also gave insights into the biosynthesis of pogostol, including stereochemical aspects of the terpene cyclisation reaction. The possible biological function of pogostol is discussed.