AbstractTwo DNA fragments (3941 and 7152 base pairs) from the procaryotic endosymbiont(Buchnera)of the aphidSchlechtendalia chinensiswere cloned and sequenced. The smaller fragment containedtrpEGand the larger fragment containedtrpDC(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 inBuchneraof the aphidSchizaphis graminum.In this endosymbiont,trpEGis amplified and located on a plasmid, whereas, in the endosymbiont ofS. chinensis, as in other eubacteria,trpEGoccurs as a single copy on the bacterial chromosome. Implications of these findings for the evolution of this mutualistic association are discusse