A feeding experiment with 13C-labeled precursors was conducted in the dual cultures of Trichoderma harzianum H14 and Catharanthus roseus callus to determine the biosynthetic origin of the carbon atoms in trichosetin. Results showed that trichosetin originated from two separate biogenetic units. One unit is an octaketide intermediate directly derived from eight intact acetate units joined in a head-to-tail fashion of a polyketide pathway. The other unit is serine.