Cloning of polyketide synthase (PKS) gene for amphidinolide biosynthesis was attempted from a dinoflagellate Amphidinium sp. (strain Y-42). Fourteen β-ketoacyl synthase gene fragments were obtained by Polymerase Chain Reaction (PCR) amplification from degenerated primer sets designed on the basis of the conserved amino acid sequences of β-ketoacyl synthase domains in known type I PKSs. The PCR analysis using primer sets designed from these fourteen β-ketoacyl synthase gene fragments revealed that these DNA sequences exist only in the dinoflagellates producing amphidinolides. The DNA sequence of the positive clone, which was isolated from genomic DNA library of Amphidinium sp. (strain Y-42) by PCR detection using the specific primer set, was analyzed by shotgun sequencing. The deduced gene products in the positive clone showed similarity with β-ketoacyl synthase (KS), acyl transferase (AT), dehydratase (DH), ketoreductase (KR), and acyl carrier protein (ACP) in known type I PKSs and thioesterase (TE).