Pseudomonas cepacia grew rapidly using a mixture of all three branched chain amino acids as carbon source, but failed to use individual branched chain amino acids as sole carbon source. Extracts of bacteria grown on branched chain amino acids had between 2-and 3-fold higher levels of ~-ketoglutaratedependent branched chain amino acid aminotransferase activity than extracts of glucose-grown bacteria. The increase in enzyme activity was due to the presence of a second aminotransferase not detected in extracts of glucose-grown bacteria. The enzyme, which presumably plays a role in branched chain amino acid degradation, had an apparent molecular weight (mol.wt.) of 75,000. The other aminotransferase was formed constitutively and apparently functions in synthesis of branched chain amino acids. It was more stable than the 75,000 mol.wt, enzyme, and was purified to homogeneity and found to be a 180,000mol.wt. oligomer containing 6 subunits of approximately 30,000 mol.wt. Antiserum prepared against the purified enzyme inhibited its activity but failed to influence the activity of the 75,000 mol.wt, aminotransferase, suggesting that the two isoenzymes are encoded by different genes.