The vole and Fuller strains of Rochalimaea quintana were grown on monolayers of mouse L cells irradiated 7 days previously and examined by light microscopy and scanning and transmission electron microscopy. Most of the bacteria of both strains were shown to adhere to the L cells but remained in an extracellular location. Cell division was frequently seen among the extracellular bacteria. The few intracellular bacteria seemed to be within vacuoles and did not multiply. Attachment to the eucaryotic cell did not seem to involve pili or other bacterial surface structures. The dimensions of the bacteria were approximately 0.45 micron in width by 1.0 to 1.7 micron in length. The cell envelope consisted of the usual trilaminar cell wall and plasma membranes separated by a layer of low electron density, as found in other gram-negative bacteria. No significant differences between the vole and Fuller strains either in morphology or relationship to eucaryotic cells were encountered.