Penicillium expansum is commonly associated with patulin production in pomaceous fruits. Both the full-length isoepoxydon dehydrogenase (idh) gene and a 470 bp fragment of the 6-methylsalicylic acid synthase (6-msas) gene have been cloned from P. expansum. In addition, we cloned a 715 bp fragment of a putative ATP-binding cassette transporter gene peab1, together with part of two putative cytochrome P450 monooxygenase genes P-450 1 and P-450 2. Increased expression of all five genes was observed under patulin-permissive conditions, indicating not only their likely involvement in patulin biosynthesis but indicating for the first time that regulation of patulin biosynthesis in P. expansum is mediated at the level of gene transcription.