Two DNA fragments (3941 and 7152 base pairs) from the procaryotic endosymbiont (Buchnera) of the aphid Schlechtendalia chinensis were cloned and sequenced. The smaller fragment contained trpEG and the larger fragment contained trpDC(F)BA, genes coding for enzymes of the tryptophan biosynthetic pathway which convert chorismate to tryptophan. Both of these gene clusters were present as one copy on the endosymbiont chromosome and probably constitute two transcription units. The deduced amino acid sequences of the proteins was 51-61% identical to the corresponding proteins were previously studied in Buchnera of the aphid Schizaphis graminum. In this endosymbiont, trpEG is amplified and located on a plasmid, whereas, in the endosymbiont of S. chinensis, as in other eubacteria, trpEG occurs as a single copy on the bacterial chromosome. Implications of these findings for the evolution of this mutualistic association are discussed.