Tissues of midguts, Malpighian tubules, salivary glands, and ovaries of female Dermacentor taiwanensis Sugimoto collected in an area endemic for Japanese spotted fever were examined with an electron microscope, and the presence of two kinds of rickettsialike microorganisms was confirmed in Japan. Based on ultrastructural findings, a rod-shaped organism detected in all the tissues is a Rickettsia; another, irregularly shaped organism that was found only in the ovary, is a Wolbachia. A hemolymph test using a specific immunostain indicated that the Rickettsia belongs to the spotted fever group of rickettsiae.