A bacterial strain was previously isolated from a parasitic wasp, Nasonia vitripennis, and shown to cause the son-killer trait in wasps. The 16S rRNA sequence, DNA probes, and whole-cell fatty acid profiles suggest that it belongs to the family Enterobacteriaceae. The strain's properties indicate a closer relationship to the genus Proteus than to the genus Escherichia, Citrobacter, or Salmonella. We propose the name Arsenophonus nasoniae gen. nov., sp. nov., for this bacterium. Strain SKI4 (ATCC 49151) is the type strain.