Flavobacterium sp. strain DS5(NRRL B-14859) was used to convert oleic acid to 10-ketostearic acid(10-KSA) via 10-hydroxystearic acid(10-HSA). Increase in cell concentration by centrifuging, collecting cells grown in two flasks, and resuspending in one flask, improved 10-KSA production to 6.5 g/L from 3.5 g/L in a usual flask culture. Tween-80 addition to the culture did not greatly affect the production of 10-KSA and 10-HSA. When culture broth was centrifuged after fermentation, it was observed that pellets were separated into two parts(yellow and white). Gas chromatography analysis showed that 10-KSA and 10-HSA were detected only in a white pellet, suggesting that the bioconversion products of strain DS5 are extracellularly produced and can be easily recovered from cells by a simple centrifugation step.