Mycoplana rubra B346 is a pink-pigmented facultatively methylotrophic bacterium which is capable of synthesizing and accumulating small amounts of poly-3-hydroxybutyric acid (PHB) if a carbon source (e.g. methanol) is provided in excess and if an essential nutrient (e.g. nitrogen or phoshorus, respectively) is limiting growth. Upon transfer of the PHA-biosynthesis genes of Alcaligenes eutrophus H16 or Chromatium vinosum D or of the isolated PHA-synthase structural gene of Methylobacterium extorquens into M. rubra B346, the content of PHB increased from 9.6 up to 25% of the cell dry matter. M. rubra and some recombinant strains were characterized physiologically and morphologically, and the size of the cells and the PHB content were compared by flow cytometry.