Citromycetin (1) has been shown by 13C n.m.r. to incorporate seven intact acetate units from [1-13C]-, [2-13C]-, and [1,2-13C2]-acetates in accordance with a polyketide biosynthesis. The distribution of radioactivity following incorporation of [2-14C]malonate was consistent with the utilisation of two starter units. The following possible advanced precursors, labelled with 14C, were not incorporated: 2,4-dihydroxy-6-methylbenzoic acid (14), 4,5,7-trihydroxyphthalide (16), and 4,7-dihydroxy-5-methyl-coumarin (18). Two compounds, 2,4,5-trihydroxy-6-methylbenzoic acid (15) and triacetic acid lactone (17), were degraded to radiolabelled acetate prior to incorporation.