Rickettsia sibirica (strain Netsvetaev) was found within large translucent spaces in the cytoplasm of L-929 cell monolayer cultures on day 6 postinfection (p.i.). These rickettsiae were rod shaped 0.3 X 10 microns in size encircled by a halo of up to 200 nm wide corresponding to a capsule-like coat. Directly on the cell wall was a 12 nm thick microcapsule in which subunit structures with 12 nm thick spacing could be recognized. The cell wall membrane was 14 nm thick with a wider internal layer occasionally in a section found split into two electron-dense lamels; the internal layer corresponded to peptidoglycan. The periplasmic space with an average thickness of 5 nm separated the cell wall from a 7 nm thick cytoplasmic membrane. The ultrastructure of R. sibirica was similar to that of other rickettsiae, although the capsule-like coat was thicker than in spotted fever (SF) group rickettsiae.