Chlamydia-like microorganism was observed not only in the cytoplasm but also within the intracytoplasmic inclusions of hepatopancreas cells of the spider, Coras luctuosus. The reticulate body, which bears a resemblance to that of members of the genus Chlamydia, is generally spherical and approximately 370 to 800mμ in diameter; its cytoplasm contains ribosome-like particles as well as fine DNA-like fibrils, and is bounded by a double-layered cell wall and a cytoplasmic membrane. The frequent occurrence of pairs or clusters of reticulate bodies and of profiles with symmetrical constriction, suggests that the reticulate bodies multiply by fission. Maturation process of the cell is accomplished by condensation of DNA-like fibrils. The elementary body is spherical, from 320 to 360mμ in diameter, with an eccentrically placed electron-dense nucleoid. In addition, four kinds of unusual parasites, which are undoubtedly cellular and procaryotic, are found only in the spider infected with the chlamydia-like microorganism. The possible relations of these parasites to the chlamydia-like microrganism are discussed.