This study was aimed at clarifying the nature of the resistance to Bacillus sphaericus Neide (Bs) that Culex pipiens L. has developed in west Mediterranean countries, France, and Tunisia. Two recessive and sex-linked mutants, sp-1^R and sp-2^R, were previously detected in southern France. Here, the Tunisian resistance was also shown to involve a single recessive and sex-linked gene that was temporarily named sp-T^R. In addition, sp-1^R, sp-2^R, and sp-T^R were shown to separately confer a similar high resistance level (>5,000-fold) in the homozygous state. Knowing that sp-1^R resistance does not alter the binding of Bs binary toxin to its specific receptor, we investigated this character in sp-2^RR and sp-T^RR homozygotes. This was performed by in vitro experiments in which larval brush border membrane fractions (BBMF) were exposed to the ¹²⁵I-Bin2 toxin of B. sphaericus strain 1593. The toxin-receptor binding was found disrupted by sp-2^R but not by sp-T^R. Comparing the binding kinetics among nine Culex pipiens strains of diverse origins revealed that the Bs receptors of sp-1^RR and sp-T^RR homozygous larvae were displaying the highest affinity toward Bs binary toxins. These results are discussed with regard to alternative assumptions on the dynamics of high Bs-resistance and on the emerging possibilities to test them in a near future.