A previously untreated field population ofCulex quinquefasciatusSay, collected near Bakersfield, CA, was subjected to intensive laboratory selection with the bacterial insecticideBacillus sphaericusNeide (strain 2362) at a level producing 95% mortality. Resistance rapidly appeared and resistance levels increased such that fourth instars of generation 12 were able to survive a concentration ofB. sphaericusthat was 7,000 times higher than the median lethal concentration (LC₅₀) of the susceptible reference colony. Similar resistance levels were detected in first instars. Cross-resistance in the selected colony was detected towardB. sphaericusstrains 1593 and 2297, but little or no cross-resistance was observed towardB. sphaericusstrains IAB59 or ISPC5 (=WHO 2173). Cross-resistance also was not detected toward the bacterial insecticideBacillus thuringiensissubsp.israelensis,toward a recombinant strain expressing bothB. thuringiensissubsp.israelensisandB. sphaericus(strain 1593) toxins, toward individual or multiple toxins fromB. thuringiensissubsp.israelensis,or toward conventional synthetic insecticides. Genetic analysis revealed thatB. sphaericusresistance was inherited as a recessive trait and controlled by a single major locus. These data are discussed in relation to cases of field resistance toward this biopesticide in theCx. pipiens(L.) complex.