Abstract
1. During late autumn and early winter of 1961 bitings of bloodthirsty mosquitoes were frequently reported as occurring in large buildings, apartment-houses and individual residences in Tokyo area. 2. It revealed that the breeding sites of these mosquitoes were temporary pools of underground rooms of large buildings and the "reserve filter section" as well as "the disinfectation section" of "septic tanks" for water-closets. 3. The progenies of these mosquitoes produced autogenous eggs. 4. The male phallosome of these mosquitoes represents the characteristic of the autogenous form "molestus" of Culex pipiens, and can be distinguished from that of Culex pipiens pallens, commonly found in Tokyo area.
