Conjugated fatty acids are regularly found in nature and have a history of biogenic activity in animals and humans. A number of these conjugated fatty acids are microbially produced and have been associated with potent anti-carcinogenic, anti-adipogenic, anti-atherosclerotic and anti-diabetogenic activities. Therefore, the identification of novel conjugated fatty acids is highly desirable. In this study, strains of bifidobacteria and propionibacteria previously shown by us and others to display linoleic acid isomerase activity were assessed for their ability to conjugate a range of other unsaturated fatty acids during fermentation. Only four, linoleic, alpha-linolenic, gamma-linolenic and stearidonic acids, were converted to their respective conjugated isomers, conjugated linoleic acid (CLA), conjugated alpha-linolenic acid (CLNA), conjugated gamma-linolenic acid (CGLA) and conjugated stearidonic acid (CSA), each of which contained a conjugated double bond at the 9,11 position. Of the strains assayed, Bifidobacterium breve DPC6330 proved the most effective conjugated fatty acid producer, bio-converting 70% of the linoleic acid to CLA, 90% of the alpha-linolenic acid to CLNA, 17% of the gamma-linolenic acid to CGLA, and 28% of the stearidonic acid to CSA at a substrate concentration of 0.3 mg mL(-1). In conclusion, strains of bifidobacteria and propionibacteria can bio-convert linoleic, alpha-linolenic, gamma-linolenic and stearidonic acids to their conjugated isomers via the activity of the enzyme linoleic acid isomerase. These conjugated fatty acids may offer the combined health promoting properties of conjugated fatty acids such as CLA and CLNA, along with those of the unsaturated fatty acids from which they are formed.