Summary: Genome reduction is commonly observed in bacteria of several phyla engaging in obligate nutritional symbioses with insects. In Actinobacteria, however, little is known about the process of genome evolution, despite their importance as prolific producers of antibiotics and their increasingly recognized role as defensive partners of insects and other organisms. Here, we show that “ Streptomyces philanthi ,” a defensive symbiont of digger wasps, has a G+C-enriched genome in the early stages of erosion, with inactivating mutations in a large proportion of genes, causing dependency on its hosts for certain nutrients, which was validated in axenic symbiont cultures. Additionally, overexpressed catabolic and biosynthetic pathways of the bacteria inside the host indicate host–symbiont metabolic integration for streamlining and control of antibiotic production. Genome and transcriptome sequencing data have been deposited in the NCBI database, BioProject, with the accession numbers [PRJNA37727][1] ([98][2]), [PRJNA542283][3] ([99][4]), and [PRJNA701492][5] ([100][6]). Proteomic data are available from the Open Research Data Repository of the Max Planck Society () ([101][7]). [1]: https://www.ncbi.nlm.nih.gov/bioproject/PRJNA37727 [2]: #ref-98 [3]: https://www.ncbi.nlm.nih.gov/bioproject/PRJNA542283 [4]: #ref-99 [5]: https://www.ncbi.nlm.nih.gov/bioproject/PRJNA701492 [6]: #ref-100 [7]: #ref-101