Probabilities of pyrethroid resistant genotypes in natural populations of Anopheles sinensis Wiedemann were measured with deltamethrin and permethrin. The median lethal concentrations (LC50s) of deltamethrin and permethrin in the susceptible larval population were 0.0209 and 0.1747 ppm, respectively. Under dosages that produced 99% mortality in susceptible laboratory strains of larvae, the lethal percentage of Cangnan larval field populations after 20 min of exposure was only 61.23% for deltamethrin and 64.92% for permethrin. This was much lower than those of other natural populations. Also, the probability of pyrethroid-resistant genotypes in Cangnan adult field populations was at the highest, reaching 0.5867. The results are discussed in relation to future mosquito control programs.