Propionic acid has been used as a cosubstrate for the production of the biodegradable polymer, poly(3-hydroxybutyric-co-3-hydroxyvaleric)acid, by Alcaligenes eutrophus. Propionic acid is, however, known to inhibit growth of A. eutrophus at concentrations above 0.5 g/l in the fermentation broth. In this study, the inhibition of the cell growth due to the presence of propionic acid was overcome by regulation of the pH. In the case of an initial pH of 7.5, A. eutrophus NCIMB 11599 continued to grow up to a propionic acid concentration of 5 g/l, whereas no growth occurred in case of an initial pH of 6.5 in a wide concentration range of propionic acid. This enhanced uptake of propionic acid with pH changes seems to be due to a decrease in the concentration of undissociated propionic acid as an uncoupler. Continuous cultures were performed in a static pH mode. When the pH was maintained at 7.5 during the fermentation, the dry cell weight was 50% higher than that when the pH was maintained at 6.8.