A transposon Tn5 mutant of Rhodobacter sphaeroides 2.4.1 was isolated for its impaired ability of growth on minimal medium containing P-hydroxybutyric acid as a sole carbon source. The mutant, R. sphaeroides S7 showed approximately 6-fold decrease in P-hydroxybutyrate dehydrogenase activity compared with that of wild type. in R. sphaeroides S7 the Tn5 was located in DNA region corresponding to a 4.2-kb EcoRI DNA fragment of R. sphaeroides 2.4.1 chromosome.