The prokaryotic endosymbiont (Buchnera) of the aphid Schizaphis graminum contains 24 copies of a plasmid that has genes encoding enzymes of the leucine biosynthetic pathway while the endosymbiont of the related aphid Diuraphis noxia has only one copy of this plasmid. These results, in conjunction with similar results for the trpEG-containing plasmids, suggest that D. noxia has a reduced demand for endosymbiont-derived essential amino acids.