Heterologous expression of four candidate genes found in ophiobolin gene clusters from three fungal strains was employed to elucidate the late-stage biosynthetic pathway of phytotoxin ophiobolin. Expression of oblB(Ac), (cytochrome P450) from the cryptic gene cluster gave unexpected products, and that of oblB(Bm)/oblB(Ev) from the gene cluster of ophiobolin producers, with oblD(Bm) as the transporter, yielded intermediate ophiobolin C through an unusual four-step oxidation process. The observation made in this study may provide a useful guideline for the elucidation of genuine biosynthetic pathways of natural products.