We detected Wolbachia infections in Culex pipiens complex mosquitoes from the Upper RhineValley, Germany, and Cebu City, Philippines, with the use of polymerase chain reaction (PCR) amplification ofthe l6S rDNA of the bacteria and further confirmation by electron microscopy. Species of 5 difl'erentautogenousand anautogenous populations of Culex pipiens fiom Germany and I population of Culex quinquefascicttas fromthe Philippines were used in this study. Larvae were sampled liom difl'erent localities and reared to the adultstage fbifurther processing. Whole adlult males and the heads of adult females were processed by PCR, whereasthe abdomens of females were used for electron microscopic study when an infection was detected by PCR. AllCulex populations assayed by PCR showed inf'ection of Wolbachia at rates between l0 and 1007o. Females fromdifTerent populations exhibitecl higher infection rates than did the males, and, likewise, Cx. pipiens populationsthat were autogenous showed higher infection rates of Wolbachia than did anautogenous individuals. In Cx.quinquefosciatzs populations, males were infected at33.337o and f'emales, only lo7o. We studied the ultrastruclureof Wolbachia in fhe ovaries of species belonging to Cx. pipiens complex by means of transmission electronmicroscopy. The bacteria exhibited typical morphology for Wolbachia with 3 enveloping membranes.