Fungal polyketides have huge structural diversity from simple aromatics to highly modified complex reduced-type compounds. Despite such diversty, single modular iterative type I polyketide synthases (iPKSs) are responsible for their carbon skeleton construction. Using heterologous expression systems, we have studied on ATX, a 6-methylsalicylic acid synthase from Aspergillus terreus as a model iPKS. In addition, iPKS functions involved in fungal spore pigment biosynthesis were analyzed together with polyketide-shortening enzymes that convert products of PKSs to shorter ketides by hydrolytic C-C bond cleavage. In our studies on reducing-type iPKSs, we cloned and expressed PKS genes, pksN, pksF, pksK and sol1 from Alternaria solani. The sol gene cluster was found to be involved in solanapyrone biosynthesis and sol5 was identified to encode solanapyrone synthase, a Diels-Alder enzyme. Our fungal PKS studies were further extended to identify the function of PKS-nonribosomal peptide synthase involved in cyclopiazonic acid biosynthesis.