Incorporation of [1-¹⁴C]acetate and [2-¹⁴C]malonate by the plant pathogenic fungus Stemphylium radicinum into the phytotoxic pyranopyrone radicinin gives a labelling pattern consistent with its formation by the condensation of two chains assembled from acetate-malonate-derived two-carbon units. [¹⁴C]Formate is incorporated mainly into carbon atoms derived from the methyl group of acetate. A co-metabolite, the antifungal macrolide pyrenophorin, is also derived from a polyketide precursor assembled by the acetate-polymalonate pathway. Some new transformation products of radicinin are described.