The International Aphid Genomics Consortium presents the draft genome sequence of the pea aphid (*Acyrthosiphon pisum*), a basal hemimetabolous insect, which provides an outgroup to holometabolous insects. The pea aphid genome is 464 Mb in size and includes extensive gene duplication in over 2000 gene families, as well as loss of evolutionarily conserved genes. Key findings include expansions in chromatin modification, miRNA synthesis, and sugar transport genes, and losses in genes central to the IMD immune pathway, selenoprotein utilization, purine salvage, and the urea cycle. The pea aphid genome reveals limited gene acquisition from bacteria, suggesting that the reduced gene count of *Buchnera* does not reflect gene transfer to the host genome. The genome also suggests extensive metabolite exchange between the aphid and *Buchnera*, including amino acid biosynthesis. This study provides a foundation for post-genomic studies of fundamental biological questions and applied agricultural problems.The International Aphid Genomics Consortium presents the draft genome sequence of the pea aphid (*Acyrthosiphon pisum*), a basal hemimetabolous insect, which provides an outgroup to holometabolous insects. The pea aphid genome is 464 Mb in size and includes extensive gene duplication in over 2000 gene families, as well as loss of evolutionarily conserved genes. Key findings include expansions in chromatin modification, miRNA synthesis, and sugar transport genes, and losses in genes central to the IMD immune pathway, selenoprotein utilization, purine salvage, and the urea cycle. The pea aphid genome reveals limited gene acquisition from bacteria, suggesting that the reduced gene count of *Buchnera* does not reflect gene transfer to the host genome. The genome also suggests extensive metabolite exchange between the aphid and *Buchnera*, including amino acid biosynthesis. This study provides a foundation for post-genomic studies of fundamental biological questions and applied agricultural problems.