Technical Abstract: Elaborate arrays of polyketide metabolites are produced by filamentous fungi. Polyketides are produced by iterative condensation of malonylCoA by a specialized type of fatty acid synthase that often lacks some or all of the reducing domains normally found in such enzymes. Fungal polyketide synthases (PKSs) have a single polypeptide chain with multiple independently acting catalytic domains. The structure of the resulting polyketide depends on the type of coenzymeA (CoA) starter unit upon which the polyketide is built, whether or not reducing domains are present in the PKS, how the polyketide chain is terminated and the types of oxidative enzymes that perform subsequent modifications. A contiguous set of genes encoding the regulatory and metabolic enzymes is usually required for the biosynthesis of polyketides. The most extensively studied biosynthetic gene cluster is for the carcinogenic mycotoxin, aflatoxin (AF). AF is formed from a hexanoyl CoA precursor by PKS-catalyzed condensation of seven malonyl CoA units and a series of oxidative steps performed by six cytochrome P450 monooxygenases, nine oxidoreductases and three non-oxidative enzymes. Since there is no evidence that these enzymes are produced sequentially, the subsequent oxidations of the polyketide precursor are most likely dictated by the chemical susceptibility of the individual precursor metabolites formed at each step.