Biosynthetic feeding experiments with singly and doubly 13C-labelled acetate show that phacidin (4-methoxy-6-nonanoyl-2H-pyran-2-one-3-carboxaldehyde) is formed from a heptaketide precursor. A similar feeding with sodium 13C-formate has shown that the methoxyl and aldehyde carbons are derived from the one-carbon pool.