Two strains of Culex pipiens molestus of different geographical origin were reared for more than five years under the same laboratory conditions in order to study to what extent the reactions of these strains to constant temperature and density become similar. Lower temperature and higher densities caused not only the prolongation of developmental time of preimaginal stages and increasing mortality of larvae and pupae in both strains but also the extension of the rate between the first and last emerging imago in both sexes of both strains. Absolute time spent in the preimaginal stage was also influenced by genetic factors. Both strains displayed different values, although both strains were for reared in the laboratory under identical conditions.