The conformation of lactarorufin B (1), a sesquiterpenoid isolated from the fruiting bodies of Russula delica, have been investigated by NMR method. Analyses of the ^1H-NMR spectra in CD_3OD at low temperatures indicate that 1 exists as a mixture of two stable conformations on the seven-membered ring in a 87 : 13 ratio which are found to be chair form and twist-boat form, respectively. The absolute stereochemistry of 1 was determined by circular dichroism (CD) spectrum
