In East Asia, 4 forms of the Culex pipiens complex have been confirmed. A form pipiens s. s. (anautogeous pipiens) has been confirmed only in westernmost China. A temperate form pallens and a subtropical and tropical form quinquefasciatus are connected with intermediates in morphology and many aspects of ecology, but their difference is rather clear in climatic adaptation traits. The distribution of a form molestus overlaps with pallens and, in Taiwan, quinquefasciatus, and, in East Asia, this form requires artificial underground habitats for its persistence. The origin and interrelation of 3 forms other than pipiens s. s. are considered from ecological aspects, especially climatic adaptation. A hypothesis is presented that molestus was originally a form having adapted to the Mediterranean climate in the western Palaearctic, secondarily colonized artificial underground habitats, and reached East Asia in the early 20th century by ships from North America. At present, it is difficult to assign pallens with certainty to either the old or new groups of the Manchurian mosquito fauna. Three hypotheses about the interrelation between pallens and quinquefasciatus are compared, and the strengths and problems of each hypothesis are indicated. Finally, a map to show the distribution of the forms of the Culex pipiens complex before it was extensively changed by humans is presented as an initial trial. The Culex pipiens problems now troubling humans are largely human-made problems.