Bioconversions of sunflower oil and rapeseed oil in fed-batch cultures fermented with Candida tropicalis M25 were studied. Cofermentations with palmitic acid resulted in successful transformations to different 3-hydroxydioic acids. The absolute configuration of the major fermentation product, R-(Z)-3-hydroxy-9-octadecenedioic acid, was determined by comparison of the H-1 NMR data with those of an authentic, optically pure compound. Using ricinoleic acid as the sole carbon source, the optically pure R-(Z)-7-hydroxy-9-octadecenedioic acid was obtained at a maximum concentration of 4.2 g/l. In addition to the hydroxydioic acid, (Z)-3,12-dihydroxy-9-octadecenedioic acid was also produced following fermentation with the mutant M25.