We succeeded in developing of a new method for in vitro poly(3-hydroxybutyrate) (P(3HB)) synthesis with coenzyme A (CoA) recycling. Thiophenyl (R)-3-hydroxybutyrate was used as a precursor for the monomer, (R)-3-hydroxybutylCoA ((R)-3HBCoA). The (R)-3HBCoA was synthesized by the ester exchange reaction between thiophenyl (R)-3-hydroxybutyrate and CoA. Although the polymerization reaction in the buffer (water phase) was performed, the reaction did not occurred. The results of a PHA synthase assay in the presence of thiophenol suggested that the reaction was inhibited by a small amount of thiophenol. To avoid the inhibition of PHA synthase by thiophenol, a water-organic solvent two-phase reaction system was developed for the polymerization reaction. The water-organic solvent two-phase reaction system has been used previously for the bioconversion of substrates with low solubility in water; it was used to remove thiophenol released during the ester exchange reaction from the water phase. Thiophenyl (R)-3-hydroxybutyrate and PHA synthase were dissolved in the organic solvent and water phases, respectively. Ester exchange between thiophenol and CoA occurs at the interface of water and the organic solvent phase, and the (R)-3HB 3-hydroxybutylCoA synthesized was polymerized sequentially by PhaC. The organic solvents with high a logP(ow) of more than 3.9 such as hexane, octane, decane, or dodecane were found to be suitable for the organic solvent phase in the two-phase system. The polymerization reaction mixture showed signs of turbidity in 2 h, after which white precipitates formed. The structural analyses by H-1 NMR spectroscopy indicated that the product was P(3HB). The weight-averaged molecular weight and molecular weight distribution of P(3HB) synthesized were found to be 1.6 x 10(6) and 2.9, respectively. The maximum yield of P(3HB) based on thiophenyl (R)-3-hydroxybutyrate was 47%.