About 300 strains of bifidobacteria were examined for their capacity to release L-isoleucine in the fermentation broth. A strain of the speciesBifidobacterium ruminale was selected as the best producer. After treatment of this strain with NTG a DL-α-aminobutyric acid-resistant mutant capable of producing about 5 mg/ml of L-isoleucine in presence of 1.5 % DL-α-aminobutyric acid was obtained. Cultural conditions and acetohydroxyacid synthetase activity were studied.