Formate-assimilating Paracoccus sp. strain 12A accumulated polyester by using formic acid as a sole carbon source without the intentional nitrogen-source limitation and restriction of air supply. The polyester produced was identified as a copolyester of P(3HB-co-5mol%3HV) by 1H and 13C NMR analysis. The molecular weight of copolyester was determined to be 6.4 x10^5 by GPC and its maximum content was 60% of dry cell weight. The strain could not assimilate 2-hydroxyctanoic acid（2HO)．By the incubation of formic acid-grown cells in the presence of 2HO for 24hr and 48hr at 30℃，however, 3HV content in P(3HB-co-5mol%3HV) increased to 20% and 41%, respectively, under nitrogen-deficentconditions. The molecular weight of these copolyester were 3.7x10^5 and 7.4x10^5,respectively. The copolyester contents of respective cells were 18.4% and 21.3% of dry cell weight. Further, the presence of many copolyester granules was observed in formic acid-grown cells by transmission electron microscopy.