Previously it has been shown that the prokaryotic endosymbiont (Buchnera) of the aphid Diuraphis noxia contains a plasmid consisting of one copy of the gene for anthranilate synthase (trpEG) and seven trpEG pseudogenes. In the present communication we show that this endosymbiont contains the remaining genes of the tryptophan biosynthetic pathway [trpDC(F)BA] which appear to be functional in that they code for the complete enzyme proteins and are not pseudogenes. As in the case of Buchnera from other endosymbionts, these genes appear to be organized as one transcription unit and are located on the endosymbiont chromosome.