Mosquito larvae of Culex pipiens were subjected repeatedly to selection pressure with the bacterial agent Bacillus thuringiensis var. israelensis (B.t.i.) in the laboratory. Only 2.78-fold increase in tolerance to B.t.i. was induced in C. pipiens as a result of 20 generations of selection. The tolerance of C. pipiens to B.t.i. decreased by about 58% after stopping the selection for three generations. Larval selection with B.t.i. caused a reduction in the reproductive potential of mosquito adult survivors but did not affect the adult longevity and the time of blood meal digestion ingested by female mosquitoes.