All Culex pipiens (wide sense) thus far examined have been infected with a rickettsia-like symbiote, Wolbachia pipientis (Hertig, 1936, Parasitology 28: 453–86), except for 2 Australian strains which were infected instead by virus (Irving-Bell, 1974, Life Sciences 14: 1149–51). Yen & Barr (1971, Nature, Lond. 232: 657–58) suggested that Wolbachia might be the cause of cytoplasmic incompatibility since this factor is inherited through the female as are the Wolbachia. These workers later (1973, Invert. Pathol. 22: 242–50) demonstrated that males “cured” of their Wolbachia infections no longer manifested incompatibility. The present note confirms this finding and presents additional information on the “curing” process.