Alcaligenes eutrophus NCIMB 11599 was cultivated to produce poly(3-hydroxybutyric-co-3-hydroxyvaleric acid), P(3HB-co-3HV), from glucose and propionic acid by a two-stage, fed-batch culture technique. The glucose concentration of the culture broth was controlled at 10-20 gl(-1) using an on-line glucose analyzer during the whole culture period. Nitrogen limitation was applied at a cell concentration of 60-70 gl(-1), when the glucose feed was replaced by the mixture of glucose and propionic acid. The effect of the ratio of propionic acid to glucose (P/G ratio) in the feed on the copolymer production was examined. The final copolymer concentrations of 117, 74, and 64 gl(-1), polymer contents of 74%, 57%, and 56.5% of dry cell weight, and productivities of 2.55, 1.67, and 1.64 gl(-1) h (-1) were obtained when the P/G ratios in the feed were 0.17, 0.35, and 0.52 (mol propionic acid mol(-1) glucose), respectively, showing reduced production with increasing P/G ratio. However, the 3-hydroxyvaleric acid (3HV) fraction in the copolymer increased with increasing P/G ratio, resulting in 14.3 mol% of 3HV at the P/G ratio of 0.52 (mol mol(-1)). The concentration of propionic acid in the culture broth was maintained below 1.3 gl(-1) when the P/G ratio in the feed was 0.17 or 0.35 (mol mol(-1)), but increased gradually when using the ratio of 0.52 (mol mol(-1)).