A continuous culture system consisting of two chemostats in series was used to investigate the kinetics of poly-3-hydroxybutyrate (PHB) formation by Ralstonia eutropha. The cells of R. eutropha grew under glucose-limited conditions in the first-stage and accumulated PHB under nitrogen-limited conditions in the second-stage. The carbon/nitrogen (CIN) ratio in the second fermenter was controlled by feeding the fermenter with a fresh glucose solution. The biokineties of R. eutropha in different nutrient conditions were studied at various steady states by controlling the dilution rates. The cell growth, glucose utilization and PHB formation under the nitrogen-rich conditions followed a Monod model very well. PHB was accumulated substantially under nitrogen-limited conditions, and the specific PHB synthetic rate was dependent on CIN molar ratio, which also closely approximated a Monod model.