Rickettsias are small, gram-negative Alpha Proteobacteria that are, with one exception, obligate intracellular parasites. They are found in a wide range of arthropods. Some of these bacteria are arthropod-vectored disease agents of vertebrates. For example, rickettsias are the causative agents of human typhus and Rocky Mountain spotted fever. Others, such as Wolbachia, are important manipulators of their arthropod host's reproductive capabilities and are not currently known to be disease agents of vertebrates.

Research on Wolbachia and related rickettsias has increased dramatically in recent years. Most of the attention has been focused on insects, although there are occasional reports of rickettsiae being found in isopods and mites. Most recently, the use of molecular techniques has established that Wolbachia is widespread in spider and predatory mites. However, Wolbachia or other rickettsias have not been demonstrated ultrastructurally in the spider mites. This report documents the presence of a rickettsia in the ovary of the adult female twospotted spider mite, Tetranchus urticae Koch, an economically important pest of ornamental and crop plants.