ObjectiveA novel biocatalyst for Baeyer–Villiger oxidations is necessary for pharmaceutical and chemical industries, so this study aims to find a Baeyer–Villiger monooxygenase (BVMO) and to improve its stability by immobilization. ResultsAcetone, the simplest ketone, was selected as the only carbon source for the screening of microorganisms with a BVMO. A eukaryote, Fusarium sp. NBRC 109816, with a BVMO ( F BVMO), was isolated from a soil sample. F BVMO was overexpressed in E. coli and successfully immobilized by the organic-inorganic nanocrystal formation method. The immobilization improved the thermostability of F BVMO. Substrate specificity investigation revealed that both free and immobilized F BVMO were found to show catalytic activities not only for Baeyer–Villiger oxidation of ketones to esters but also for oxidation of sulfides to sulfoxides. Furthermore, a preparative scale reaction using immobilized F BVMO was successfully conducted. ConclusionsFBVMO was discovered from an environmental sample, overexpressed in E. coli , and immobilized by the organic-inorganic nanocrystal formation method. The immobilization successfully improved its thermostability.