Mendelian crosses were used to study the mode of inheritance of Cry toxin resistance in a Culex quinquefasciatus Say (Diptera: Culicidae) colony (CqAB11A) that evolved insecticide resistance under laboratory selection with a deletion mutant of Bacillus thuringiensis subsp. israelensis de Barjac lacking the Cyt1Aa toxin component but containing its three major Cry toxins, Cry4Aa, Cry4Ba, and Cry11Aa. High levels of resistance were observed to Cry toxins. F1 offspring of reciprocal crosses to a sensitive colony showed intermediate levels of resistance with no maternal effect, indicating autosomal inheritance. Dose—response data for backcross offspring deviated significantly from the monofactorial model when tested with Cry4Aa Cry4Ba Cry11Aa, Cry4Aa Cry4Ba, or Cry11Aa. However, tests with Cry11Ba from B. thuringiensis subsp. jegathesan (Seleena, Lee, Lecadet) fit the monofactorial model. Dominance of F1 offspring was calculated at different concentrations of Cry-toxin suspensions and, as reported for other Cry-resistant Culex, generally decreased as concentration increased. A subset of colony CqAB11A was reared without selection pressure for 18 generations with little change in susceptibility, indicating a highly homozygous population. Consistent with reports for other Cry-resistant Culex, the data show these mosquitoes evolved resistance to B. thuringiensis Cry toxins at multiple loci in response to selection pressure and that cross-resistance to Cry11Ba was conferred by one of those loci.