Pseudomonas putida KT2440 grew on glucose at a specific rate of 0.48 h(-1) but accumulated almost no poly3- hydroxyalkanoates (PHA). Subsequent nitrogen limitation on nonanoic acid resulted in the accumulation of only 27% medium-chain-length PHA (MCL-PHA). In contrast, exponential nonanoic acid-limited growth (mu = 0.15 h(-1)) produced 70 g l(-1) biomass containing 75% PHA. At a higher exponential feed rate (mu = 0.25 h(-1)), the overall productivity was increased but less biomass (56 g l(-1)) was produced due to higher oxygen demand, and the biomass contained less PHA (67%). It was concluded that carbon-limited exponential feeding of nonanoic acid or related substrates to cultures of P. putida KT2440 is a simple and highly effective method of producing MCL-PHA. Nitrogen limitation is unnecessary.