The article reports the complete genome sequence of *Buchnera* sp. strain APS, a symbiotic bacterium found in the bacteriocytes of aphids. *Buchnera* is a close relative of *Escherichia coli* but has a significantly smaller genome, containing only one 640,681-base-pair chromosome and two small plasmids. The genome lacks genes for non-essential amino acid biosynthesis and cell-surface components, indicating a highly specialized and symbiotic relationship with its host. The study highlights the complementary nature of the symbiosis, where *Buchnera* provides essential amino acids to the host, while the host provides precursors for some of these amino acids. The genome also lacks genes for DNA repair, recombination, and regulatory systems, suggesting a limited ability to respond to environmental changes. The evolutionary analysis indicates that the small genome of *Buchnera* is the result of reductive evolution, reflecting its obligate mutualistic relationship with its host. This research provides insights into the genomic evolution of mutualistic organisms and the unique adaptations required for intracellular symbiosis.The article reports the complete genome sequence of *Buchnera* sp. strain APS, a symbiotic bacterium found in the bacteriocytes of aphids. *Buchnera* is a close relative of *Escherichia coli* but has a significantly smaller genome, containing only one 640,681-base-pair chromosome and two small plasmids. The genome lacks genes for non-essential amino acid biosynthesis and cell-surface components, indicating a highly specialized and symbiotic relationship with its host. The study highlights the complementary nature of the symbiosis, where *Buchnera* provides essential amino acids to the host, while the host provides precursors for some of these amino acids. The genome also lacks genes for DNA repair, recombination, and regulatory systems, suggesting a limited ability to respond to environmental changes. The evolutionary analysis indicates that the small genome of *Buchnera* is the result of reductive evolution, reflecting its obligate mutualistic relationship with its host. This research provides insights into the genomic evolution of mutualistic organisms and the unique adaptations required for intracellular symbiosis.