Comparative studies were conducted on Culex perexiguus and Cx. neavei, two members of the medically important univittatus complex, to determine if they represent the same or different species. Specimens of neavei from countries south of the Sahara were compared with specimens of perexiguus from countries in northern Africa and southwestern Asia. Slight and variable differences were observed in the ornamentation of adults, males differed constantly in a single feature of the genitalia, and several tenuous distinctions were noted in larvae. A high degree of unidirectional incompatibility occurred when neavei females from South Africa were mated with perexiguus males from Egypt. It is concluded that perexiguus and neavei represent different, largely allopatric species.