While catabolism of amino acids is believed to play an important role in cheese flavor development, the pathways present in cheese microflora are poorly understood. To determine the pathways of aromatic amino acid catabolism in lactococci and effects of Cheddar cheese ripening conditions on catabolic enzymes and products, eight starter lactococcal strains were screened. Cell-free extracts prepared from these strains were found to contain an α-ketoglutarate-dependent aminotransferase activity with tryptophan, tyrosine and phenylalanine. Tryptophan, tyrosine and phenylalanine aminotransferase specific activities (μmol product formed / mg protein / min) ranged from 0.30 to 2.8 10-3, 0.93 to 7.3 10-3 and 1.5 to 7.2 10-3, respectively. Metabolites produced from tryptophan by a cell-free extract of Lactococcus lactis S3 were indolepyruvic acid, indoleacetic acid and indole-3-aldehyde. Indoleacetic acid and indole-3-aldehyde can form spontaneously from indolepyruvic acid under the conditions employed. A defined medium was used to determine whether the aminotransferase(s) was expressed and which metabolite(s) accumulate under conditions that simulated those of ripening Cheddar cheese in terms of pH, salt, temperature and carbohydrate starvation. The results indicated that the aminotransferase(s) was expressed and stable under these conditions. The tryptophan metabolites that accumulated were determined to be strain-specifïc.