RICKETTSIAE multiply by binary fission; this has been clearly shown for R. mooseri, R. tsutsugamushi, R. rickettsii, R. quintana and R. orientalis1–4. The multiplication of Coxiella burneti is, however, more problematical. The virus-like character of these organisms has often been described5–9. It has often been suggested that there is an eclipse phase; that there is active participation of the cell in reduplication (alterations in the cytoplasm and the formation of different inclusions), and that there are intermediate forms and infectious ultrafilterable particles. Study of Coxiella in yolk sac suspensions has, however, revealed binary fissions10. We are studying the development of these organisms in guinea-pig testes, and have also found signs that this type of multiplication is occurring.