In southern France, failure to control Culex pipiens L. with Bacillus sphaericus Neide toxin (Bs) was first detected in 1994, at the extreme east of the Languedoc-Roussillon coast. This failure was due to a single recessive mutant, sp-1^R. Two complementary strategies were used to test whether sp-1^R had invaded the Bs-controlled area by 1998. First, a strain (BP) was selected from resistant larvae sampled in the western part of the Bs-controlled area. In BP strain, resistance involved a single recessive gene, sp-2^R, distinct from sp-1^R, that conferred a similarly high resistance in the homozygous state (≈6,000-fold). Combining one copy of sp-1^R and one of sp-2^R conferred a > 100-fold resistance. Second, Bs-resistance was monitored among the offspring of field females crossed to sp-1^RR homozygous males. Females were sampled in 20 localities of southern France and three localities of the Llobregat delta (Barcelona, Spain) where C. pipiens control is also intensive. The 537 females in the study produced enough larvae to infer their genotype: 462 progenies were susceptible and the survival rate of 51 others was explained by the presence of sp-1^R and/or sp-2^R. The remaining 24 cases indicated that other factors could confer resistance when combined with sp-1^R. The current data showed that, even when recessive, resistant mutants can rapidly increase in frequency, providing some interactions that protect them from disappearance. We discuss the consequences of this finding on the current strategies aimed to avoid or delay resistance in the pests controlled with B. sphaericus or B. thuringiensis Berliner toxins.