The effect of glucose and NH4+ concentrations on poly(beta-hydroxybutyrate) (PHB) production was investigated with an NH4+:pH-auxostat. Reactor concentrations were controlled between 1.5 mu M and 10 mM for NH4+ and between 1 and 20 g/L for glucose. Dilution rates between 0.07 and 0.2 h(-1) were achieved, but little correlation was observed between nutrient concentrations or dilution rate on PHB concentration, which varied between 10 and 20% of the cell dry weight. However experiments demonstrated that glucose levels above 1 g/L affected the growth rate when the culture was NH4+-limited, implying a non-Monod growth model. The residual biomass yield on glucose was estimated to be 0.5 g of cells/g of glucose and was independent of dilution rate. However, the estimated yield on NH4+ decreased linearly with increasing dilution rate between 9 and 6 g of cells/g of NH4+. A qualitative model for PHB formation based on the cellular response to the environmental stress of low NH4+ concentrations is discussed.