Cultured cells of the liverwort, Heteroscyphus planus, accumulate a wide variety of sesquiterpenes and diterpenes of the clerodane type. ¹³C Atoms of [2-¹³C]acetate and [2-¹³C]- and [4,5-¹³C₂]-mevalonates (MVAs) are incorporated into sesquiterpenes more extensively than into diterpenes by cultured cells of H. planus. (1S)-7-Methoxy-1,2-dihydrocadalene, a predominant sesquiterpene, and heteroscyphic acid A, a main diterpene, incorporating ¹³C labelled precursors were analysed by ¹³C NMR spectroscopy. Preferential labelling of the farnesyl diphosphate (FPP)-derived portion of heteroscyphic acid A was confirmed by ¹³C enriched peaks and ¹³C–¹³C couplings, while sesquiterpenes were labelled equivalently. From these results, it appears that separate sites for sesquiterpene and diterpene biosynthesis exist in cultured cells of H. planus as could be observed in higher plants. Since the same type of nonequivalent labelling was observed in the phytyl side chain of chlorophyll by other experiments, it is suggested that all compounds, which are formed from GGPP within chloroplasts, are biosynthesized partly via the condensation of FPP derived from exogenous MVA with endogenous isopentenyl diphosphate in chloroplasts. A 1,2-methyl migration of the methyl group originating at C-3 of MVA is also confirmed in the biosynthesis of the trans-clerodane skeleton.