Ehrlichia sennetsu, the etiologic agent of human sennetsu rickettsiosis was successfully propagated in a continuous cell culture using murine cell lines P388D1 and Raw 264. Pleomorphic cytoplasmic inclusion bodies similar to Ehrlichia canis morulae were observed 3-4 days after second post-inoculation split. In the Raw 264 cell line E. sennetsu was not seen until the third passage. Relatively heavier infection was observed in P388D1 than in Raw cell line. The latter reached a maximum of 15% infection whereas P388D1 cell line attained saturation. Structural details of the organism were confirmed by electron microscopy. A unique rippled cell mass surrounding the plasma membrane was observed. Supernatants of cultures were shown to contain infectious organisms. The advantages of propagating E. sennetsu in continuous cell lines are discussed with respect to future physiochemical and immunochemical studies of this organism.