Production of medium-chain-length (MCL) poly(3-hydroxyalkanoates) (PHAs) from a strain of Pseudomonas putida was studied. Although octanoate was found to be efficient for MCL PHA production, its yield for PHAs was low since it was also used for cell growth. A two-step fed-batch cultivation was thus designed with glucose and octanoate as the main carbon source for cell growth and PHA accumulation, respectively. After the cells were grown on glucose in the first step, they were subjected to nitrogen- and oxygen-limiting conditions in the second step in which octanoate was supplied as the sole carbon source. PHA (18.6 g l(-1)) was obtained with a yield of about 40% (g PHA g(-1) octanoate). By using the same method but supplying octanoate in the first cell-growing step, production of MCL PHAs was significantly enhanced; it yielded 35.9 g PHA l(-1) after 39 h of the fed-batch operation. The PHA content of the cell was also improved to 65.5% (g PHA g(-1) cell mass) which indicated that octanoate addition stimulated quite efficiently the biosynthesis of MCL PHAs.