In our previous paper, we synthesized poly-3-hydroxybutyrate [P(3HB)] by using the water-organic-solvent two-phase reaction system (TPRS), in which (R)-3-hydroxybutyrylCoA [(R)-3HBCoA] was continuously supplied to PHA synthase by the ester exchange reaction between CoA and thiophenol in thiophenyl (R)-3HB [(R)-3HBTP]. By applying TPRS to the screening, we found a lactate- (LA-) polymerizing enzyme from PHA synthases. The enzyme was an engineered PHA synthase, which stereoselectively copolymerized (R)-LA together with (R)-3HB. NMR and GPC revealed that the TPRS successfully synthesized poly(lactate-co-(3-hydroxybutyrate)) [P(LA-co-3HB)] using the LA-polymerizing enzyme as a catalyst. The molar ratios of LA in the copolymers were controllable in the range of 0 to 36 mol% by varying the ratio of (R)-LATP and (R)-3HBTP fed into the TPRS. The number-average molecular weight and the polydispersity of P(36 mol%-co-3HB) were 1.1 x 10(4) and 1.4, respectively. This is the first report on the chemo-enzymatic synthesis of P(LA-co-3HB) by a LA-polymerizing enzyme.