Microbial degradation of Trp is thought to promote the formation of aromatic compounds that impart putrid, fecal, or unclean flavors in cheese, but pathways for their production have not been established. This study investigated Trp catabolism by Lactobacillus casei and Lactobacillus helveticus cheese flavor adjuncts under carbohydrate starvation (pH 6.5, 30 or 37°C, no sugar) and near cheese-ripening (pH 5.2, 4% NaCl, 15°C, no sugar) conditions. Enzyme assays of cell-free extracts indicated that both species of Lactobacillus catabolized Trp to indole-3-lactic acid, and micellar electrokinetic capillary chromatography of culture supernatants showed this reaction occurred via successive transamination and dehydrogenation reactions. Tryptophan decarboxylase activity was also detected in all Lactobacillus cell-free extracts, but tryptamine was not detected in culture supernatants. Micellar electrokinetic capillary chromatography showed that Trp metabolism in Lactobacillus casei LC301 and LC202 was similar under both incubation conditions and that those catabolic reactions were reversible (i.e., conversion of indole-3-lactic acid to Trp). In contrast, Trp catabolism by Lactobacillus helveticus LH212 was only detected under near cheese ripening conditions. Cells of Lactobacillus helveticus CNRZ32 did not catabolize Trp in either condition but did convert indole-3-pyruvic acid to Trp in carbohydrate starvation medium and to Trp and indole-3-lactic acid under near cheese ripening conditions.