Agarsenone (1), a new cadinane sesquiterpenoid, was isolated from the resin of Commiphora erythraea. The structures of 1 and its decomposition products agarsenolides (2a and 2b) and myrrhone (3) were established by extensive NMR spectroscopic analysis. The absolute configuration of 3 and the relative and absolute configurations of 1 were assigned by comparison of experimental and calculated optical rotatory dispersion and electronic circular dichroism spectra.