Mendelian crosses were used to analyze the patterns of inheritance of Cry-toxin resistance in two colonies of Culex quinquefasciatus Say larvae resistant to bacterial toxins produced by Bacillus thuringiensis subsp. israelensis de Barjac. Resistance levels exceeded 1000-fold at 95% lethal concentration of the Cry11Aa-resistant colony (Cq11A). F₁ offspring of reciprocal crosses to a susceptible colony revealed autosomal inheritance and offspring were intermediate in resistance to the susceptible and resistant parental lines. Dose-response tests on backcross offspring were consistent with polyfactorial inheritance of resistance toward Cry11Aa and Cry4Aa Cry4Ba, whereas cross-resistance toward Cry11Ba best fit a monofactorial model. Resistance was 600-fold at 95% lethal concentration in the colony selected with Cry4A Cry4B (Cq4AB). Inheritance of resistance in F₁ offspring was autosomal and intermediate to the susceptible and resistant parents. Inheritance of Cry4Aa Cry4Ba and Cry11Ba resistance best fit a polyfactorial model in offspring of the Cq4AB backcross, whereas Cry11Aa-resistance inheritance fit a monofactorial model. Dominance values were calculated at different Cry-toxin concentrations for F₁ offspring of both resistant colonies; dominance generally decreased as treatment concentration increased. Resistance and cross-resistance remained stable in Cq11A and Cq4AB in the absence of insecticide pressure. Allelic complementation tests were complementary and suggested that Cq11A and Cq4AB evolved resistance to Cry toxins at common loci. The patterns of cross-resistance suggest cross-recognition of binding moieties by Cry11Aa, Cry4Aa Cry4Ba, and Cry11Ba in these Culex, which may be partly responsible for the toxin synergy characteristic of B. thuringiensis subsp. israelensis de Barjac.