Microorganisms with high activity of linoleic acid delta12-cis,delta11-trans-isomerase were isolated from the digestive tract of ruminants and characterized. The isolate with the highest isomerase activity was identified as Pseudobutyrivibrio ruminis. The susceptibility of this strain to 3 fatty acids added to the grow medium was determined. A significant inhibition of bacterial growth (during a 3-d period) by linoleic acid (0.1 %) and oleic acid (5 ppm) was observed; no inhibition was found in the presence of stearic acid.