Two inexpensive substrates, starch and whey were used to produce poly(3-hydroxybutyrate) (PHB) in fed-batch cultures of Azotobacter chroococcum and recombinant Escherichia coli, respectively. Oxygen limitation increased PI-IB contents in both fermentations. In fed-batch culture of A. chroococcum, cell concentration of 54 g l(-1) with 46% PHB was obtained with oxygen limitation, whereas 71 g l(-1) of cell with 20% PHB was obtained without oxygen limitation. The timing of PHB biosynthesis in recombinant E. coli was controlled using the agitation speed of a stirred tank fermenter. A PHB content of 80% could be obtained with oxygen limitation by increasing the agitation speed up to only 500 rpm. (C) 2000 Elsevier Science Inc. All rights reserved.