The purine salvage pathway recycles purines to nucleotides, promoting efficient utilization of purine nucleotides. Exceptionally among animals with completely sequenced genomes, the pea aphid lacks key purine recycling genes that code for purine nucleoside phosphorylase and adenosine deaminase, indicating that the aphid can neither metabolize nucleosides to the corresponding purines, nor adenosine to inosine. Purine metabolism genes in the symbiotic bacterium Buchnera complement aphid genes, and Buchnera can meet its nucleotide requirement from aphid-derived guanosine. Buchnera demand for nucleosides may have relaxed the selection for purine recycling in the aphid, leading to the loss of key aphid purine salvage genes. Further, the coupled purine metabolism of aphid and Buchnera could contribute to the dependence of the pea aphid on this symbiosis.