The definition of genus Rickettsiella given by PHILIP should be completed and corrected because in all instars of insects particularly the cytoplasm, not the nucleus of the diseased cells was infected. No morphological differences were found between the known types Rickettsiella popilliae, Rickettsiella melolonthae and Rickettsiella tipulae. In serological experiments a group-specific partial-antigen and a type-specific partial-antigen were demonstrated in each of the studied Rickettsiellae.