Technical Abstract: A new microbial isolate, Flavobacterium sp. DS5 converted oleic and linoleic acids to their corresponding 10-keto- and 10-hydroxy fatty acids. The hydration enzyme seems to be specific to the C-10 position. Conversion products from alpha- and gamma-linolenic acids were identified by GC/MS, FTIR and NMR as 10-hydroxy-12(Z),15(Z)-octadecadienoic and 10-hydroxy-6(Z),12(Z)-octadecadienoic acids, respectively. Products from other 9(Z)-unsaturated fatty acids were also identified as their corresponding 10-hydroxy and 10-keto-fatty acids. Trans unsaturated fatty acid was not converted. From these results, it is concluded that strain DS5 hydratase is indeed a C-10 positional specific and cis-specific enzyme. DS5 hydratase prefers a 18 carbon monounsaturated fatty acid. Among the C18 unsaturated fatty acids, additional double bonds at either side of the 9,10 position lower the enzyme hydration activity. The fact that hydratases from other microbes also convert 9(Z)-unsaturated fatty acids to 10-hydroxy fatty acids, the C-10 positional specificity of microbial hydratases might be universal.