Maternally inherited bacteria that kill male but not female hosts during embryogenesis occur in Harmonia axyridis (Pallas). In two populations in Japan, Fukuyama, Hiroshima, and Muikamachi, Niigata, H. axyridis were infected with male-killing bacteria. According to the sequence analysis of the gene fragment for 16S rDNA, these bacteria belong to the genus Spiroplasma. Tetracycline-treated infected females produced female and male progeny, although untreated females produced only female progeny, demonstrating that H. axyridis is male-killed by the Spiroplasma. The proportion of females infected in Muikamachi and Fukuyama was 0.039 and 0.135, respectively, indicating that prevalence of male-killer in H. axyridis is low in Japan.