The synthesis of poly(3-hydroxyalkanoates) (PHA) by Pseudomonas putida KT2442 growing on long-chain fatty acids was studied in continuous cultures. The effects of the growth rate on the biomass and polymer concentration were determined and it was found that the PHA concentrations decreased with increasing growth rates. The highest volumetric productivity was 0.13 g PHA l(-1) h(-1) at a specific growth rate (mu) Of 0.1 h(-1). The molecular mass of the polymer remained constant at all growth rates but changes in the monomeric composition of the PHA synthesized were observed. Variation of the carbon to nitrogen (C/N) ratio of the substrate feed at mu = 0.1 h(-1) revealed optimal PHA formation at C/N = 20 mol/mol. In order to optimize PHA production P. putida KT2442 was cultivated to high cell densities in oxygen-limited continuous cultures. Tn this way a maximum biomass concentration of 30 g/l containing approximately 23% PHA was achieved. This corresponds to a volumetric productivity of 0.69 g l(-1) h(-1).