Fed-batch culture of Pseudomonas oleovorans was carried out for the production of medium-chain-length polyhydroxyalkanoates (MCL-PHAs) using octanoate as a carbon source. Octanoate and the salt solution containing ammonium sulfate and magnesium sulfate were intermittently fed in the course of fermentation. Cell mass and PHA concentrations of 42.8 and 16.8 g/L, respectively, could be obtained in 40 h. The PHA content and the PHA productivity were 39.2% and 0.42 g PHA/L-h, respectively. The yields of cell mass and PHA were 0.71 g dry cell mass/goctanoate and 0.28 g PHA/g octanoate, respectively. Therefore, octanoate can be used for the production of MCL-PHAs to a high concentration with high productvity.