The transformation of oleic acid by mutants of Candida tropicalis was studied in fed-batch cultures. Besides Delta(9)-cis-1,18-octadecenedioic acid, 3-hydroxy-Delta 9-cis-1,18-octadecenedioic acid was detected as the main fermentation product. Here we describe the production, isolation and the complete chemical characterization of the purified 3-hydroxy-Delta(9)-cis-1,l8-octadecenedioic acid. The geometric configuration of the double bond was not changed during bioconversion. The enantiomeric excess of the compound was 76%. Mutagenesis of C. tropicalis DSM 3152 with N-methyl-N-nitro-N’-nitrosoguanidine and selection with oleic acid as the sole carbon source led to mutant M 25, which produced the 3-hydroxy-Delta(9)-cis-1,18-octadecenedioic acid at a 1.8-fold higher concentration in the medium as compared to the parent strain. The maximum concentration of the hydroxy dioic acid was 19.4 g/l after 223 h fermentation.