In 70 natural populations and laboratory strains of Culex pipiens from the former Soviet Union (32 settlements, 37-62 N, 38-93 degrees E) the larval siphonal index was studied, in 29 populations the adult biological peculiarities (the autogeny, the stenogramy and the bloodsucking activity on the human) were studied also in the laboratory. The individual siphonal index varies considerably from 2.2 to 7.9 and has the normal distribution shape. The mean index is correlated with the larval biotope (open or underground reservoirs) and with biological peculiarities. In autogenous and stenogamous populations the mean siphonal index varies from 3.7 to 4.6 (average 4.1), in nonautogenous and eurigamous ones--from 4.7 to 5.6 (average 5.1). The biological mechanisms which provide the differentiation and isolation of the forms C. p. pipiens and C. p. molestus in the temperate zone are discussed. These mechanisms include the capacity to diapause, the ability to copulate in the restricted space (stenogamy), and the biotopic specialization of the larvae (open or underground reservoirs). The individual variation of siphonal index is often too high, therefore the identification of the individual larvae is unreliable. However, the mean siphonal index of the population can be recommended for the identification of the forms in populations from the temperate latitudes: in C. p. molestus the mean siphonal index is 4.3 and lower, in C. p. pipiens it is 4.8 and higher, the rare populations with the intermediate indexes require the additional biological study.