A rickettsiella-like organism described in the spider P. mirabilis Cl., was placed in the genus Rickettsiella on the basis of its cycle and ultrastructure. A chlamydial cycle takes place in the digestive cells and becomes more complex in the interstitial cells where giant bodies appear. The latter have been considered up to now as characteristic of Rickettsiella but seem related to the metabolism of the parasitized cell. Their presence and structure can no longer be regarded as specific characteristics. A study of the host and tissue specificity of the various known Rickettsiella must be defined before the impact of the metabolism of host cells on the appearance and structure of these bodies can be determined.