The copolyester of 3-hydroxybutyrate and 3-hydroxyvalerate was synthesized from the combined carbon sources of glucose and sodium propionate by a filamentaion-defective mutant of Sphaerorilus natans, which is a typical filamentous bacterium often found in activated sludge. The 3-hydroxyvalerate content in the produced polymer increased with increasing, concentrations of propionate. Cell growth and polyester synthesis were observed even when 0.6% sodium propionate was added to the medium : when the 3-hydroxyvalerate content in the polymer produced was about 60 mol%. The monomer composition of the copolymer was also varied by aeration conditions, time of propionate feeding, and cultivation time. This strain flocculated in accordance with cell growth, allowing rapid and convenient separation of the biomass from the culture fluid.