Bio-process development for isomer selective and efficient production of cis-9,trans-11-octadecadienoic acid (CLA) from trans-vaccenic acid (t-VA, trans-11-octadecenoic acid) through microbial fatty acid Delta 9-desaturation reaction. A total of 550 strains of fungi and yeasts were screened for CLA production from t-VA through Delta 9 desaturation. Delacroixia coronata IFO 8586 was selected as a potent producer of CLA from t-VA. Efficient CLA production was observed during cultivation in medium supplemented with the methyl ester of t-VA (t-VAME). Under the optimal conditions with 33.3 mg ml(-1) of t-VAME as substrate, 10.5 mg ml(-1) CLA was produced by D. coronata IFO 8586 after 7 days of cultivation in the medium containing dextrin (5.0%), tryptone (2.0%) and thiourea (0.83 mu mol ml(-1)). The strain produced the cis-9,trans-11 isomer of CLA selectively (98% of total CLA), with a small amount of the trans-9,trans-11 isomer (2% of total CLA), mainly in the form of triacylglycerols (69% of total CLA). A practical bio-process for selective production of cis-9,trans-11 isomer of CLA using filamentous fungus D. coronata IFO 8586 was successfully established. Isomer selective bio-process for the practical production of cis-9,trans-11-CLA was first established. The process is benefitable for expanding the application of CLA for medicinal and nutraceutical purposes.