The molecular weight, sedimentation coefficient, and amino acids composition were determined on five tryptophanases (TPases) from Escherichia coli B and E. aurescens, Shigella alkalescens, and Proteus vulgaris and P. morganii. These TPases have identical sedimentation profile and coefficient (9.6 S), and the same molecular weight (220 000). Each enzyme is constituted of four identical subunits having a molecular weight of 55 000. The amino acids composition of these TPases is very similar, with the exception of P. morganii and P. vulgaris TPases which present significative variations in basic amino acids and tryptophan content. The species differentiation of the coli group cannot be made on their TPase characteristics only, contrary to P. morganii and P. vulgaris which can be differentiated between them and from the coli group.