Effects of propionate on the accumulation of poly(beta-hydroxybutyrate-co-beta-hydroxyvalerate) and the excretion of pyruvate in Alcaligenes eutrophus were investigated at various concentrations of glucose and propionate. As propionate concentration increased, an enhancement in pyruvate excretion was observed along with a decrease in the yield of the copolymer. At the same concentration of propionate; hydroxyvalerate content of the copolymer was reduced from 26 to 15 mol % with increase of the initial glucose concentration.