Cytoplasmic incompatibility (CI) in insects is an intrapopulational sterility phenomenon. Although known for nearly 40 years, it has only recently attracted the attention of evolutionary biologists, having been found in an increasing number of species. Apparently, the proximate cause of CI is the presence of rickettsia-like endocellular microorganisms. An ultimate cause can be identified in the fact that males do not transmit cytoplasmic genes, including those of rickettsia.