Stereoselectivity in the oxidation of ferrocenyl sulphides with several species of microorganisms such as Corynebacterium equi was reversed between (methylthiomethyl)ferrocene and (t-butylthiomethyl)ferrocene, indicating that the size of the side chain could control the stereochemical course of the biooxidation. The size seems to be effective within some limit, because the more bulky compound, (phenylthiomethyl)ferrocene, was a poor substrate. On the other hand, the microbial oxidation of (methylthiomethyl)ruthenocene and -osmocene with Mortierella isabellina and that of (t-butylthiomethyl) ruthenocene and -osmocene with C. equi gave levorotatory sulphoxides (85-97% e.e.) : which were all found to have R absolute configuration. These stereoselectivities were the same as those for the corresponding ferrocenyl sulphides. Thus, the central atom of the metallocenic sulphides seems not to affect the stereoselectivity in the biooxidation. The microbial oxidation is a useful method to prepare several kinds of chiral metallocenic sulphoxides with high optical purity.