A fragment of plasmid NAH7 from Pseudomonas putida PpG7 has been cloned and expressed in Escherichia coli HB101. Growth of the recombinant Escherichia coli in nutrient medium results in the formation of indigo. The production of this dye is increased in the presence of tryptophan or indole. Several bacteria that oxidize aromatic hydrocarbons to cis-dihydrodiols also oxidize indole to indigo. The results suggest that indigo formation is due to the combined activities of tryptophanase and naphthalene dioxygenase.