The cytochrome P450 enzyme RosC catalyzes a two-step, hydroxylation and alcohol oxidation, oxidation reaction to form the C-20 formyl group in the biosynthesis of a 16-membered macrolide antibiotic rosamicin produced by Micromonospora rosaria IFO13697. RosC is presumed to be involved in the formation of 20-carboxyrosamicin because it has been isolated from the culture broth of M. rosaria. Here, we confirmed that RosC has catalytic activity, with E. coli expressing RosC converting rosamicin into 20-carboxyrosamicin. Therefore, it was revealed that RosC is a multifunctional P450 that catalyzes a three-step oxidation reaction that leads to the formation of the hydroxyl group, formyl group and carboxyl group at C-20 on the macrolactone ring in the rosamicin biosynthetic pathway. Moreover, the cytochrome P450 enzyme TylI, which is involved in formation of the formyl group of a 16-membered macrolide antibiotic tylosin produced by Streptomyces fradiae ATCC 19609, also converted rosamicin into 20-carboxyrosamicin.