The conversion of linoleic acid into 10-hydroxy-12(Z)-octadecenoic acid by whole cells of Stenotrophomonas nitritireducens as an isolated bacterium was optimized, and the optimal temperature, pH, and cell and substrate concentrations were 30 degrees C, 7.5, and 20 and 20 g/L, respectively. Under these conditions, whole cells in a bioreactor produced 15 g/L 10-hydroxy-12(Z)-octadecenoic acid in 2 h of reaction time without detectable byproducts. Using 2 g/L linoleic acid, the cells produced 1.92 g/L 10-hydroxy-12(Z)-octadecenoic acid. These are the highest concentration and yield of 10-hydroxy-12(Z)-octadecenoic acid ever reported.