Chalcomoracin (1), produced in Morus alba L. cell cultures at high levels, is a phenolic compound bearing two hemiterpene moieties in the molecule. Administration of D-[U-C-13(6)]glucose to the cell cultures provided 1 labeled with C-13. Analogous experiments with [1,3-C-13(2)]- and [2-C-13]glycerol demonstrated that the C-13-labeling pattern in the second and third acetate units for the mevalonate pathway was reversed. This reversed C-13-labeling pattern suggests that, in the pair of three acetate units composing the two hemiterpene moieties of 1, the starter acetate unit comes directly from glycolysis, while the second and third acetate units originate from the pentose phosphate cycle.