The following indole compounds have been unequivocally identified and semi‐quantitated in culture supernatants of Rhizobium strains by gas chromatography mass spectrometry: indole‐3‐aldehyde, trytophol, indole‐3‐carboxylic acid, indole‐3‐glycollc acid, indole‐3‐actic acid, indole‐3‐glyoxylic acid, N‐acethyl‐L‐tryptophan and indole‐3‐pyruvic acid, as well as indole‐3‐acetic acid. The strains included a number of chosen Rhizobium mutants defective in various stages of nodule formation, and the biological implications of the findings are discussed. Two alternative approaches to sample purification were taken, one based primarily on solvent partitioning and the other on high‐performance liquid chromatography. The advantages of each method are discussed. Gas chromatographic retention times and mass spectra are given for the TMS derivatives of 17 authentic indole standards.