Aim: Isolation of a new strain of Butyrivibrio fibrisolvens possessing great capacity to produce conjugated linoleic acid (CLA) in order to utilize as a probiotic for animals. Methods and Results: A novel strain (MDT-5) was isolated from the goat rumen, which exclusively converted linoleic acid (LA) to CLA, because of its high LA isomerase activity with virtually no CLA reductase activity. MDT-5 also converted linolenic acid to conjugated linolenic acid that may be more bioactive than CLA. The oral administration of MDT-5 every other day to mice for 2 weeks resulted in increased amounts of CLA in the contents of the large intestine (2.5-fold), as well as in adipose tissue (threefold). Feeding a high-LA diet, as well as prolonging the period of MDT-5 administration, further increased the CLA content in body fat. Conclusions: MDT-5 has by far greater ability to produce CLA than any other known bacteria. Administration of MDT-5 to mice increases CLA production in the large intestine, which results in increased CLA absorption. Significance and Impact of the Study: MDT-5 may be useful in pet animals as a probiotic to provide CLA continuously.