Toxicity of malathion, profenophos, cypermethrin, fenvalerate, methomyl, propoxure, spinosad and abamectin was tested against larvae of laboratory (S) and three field (AM, AU and W) strains of C. pipiens (L). Based on LC₅₀ values, spinosad was the most toxic compound against the S strain (LC₅₀ = 0.0156 ppb), while fenvalerate and cypermethrin were the most effective insecticides against the three field populations. Values of  LC₅₀ for fenvalerate for AM, AU and W strains were 0.497, 0.315 and 0.868 ppb, respectively, and the corresponding values for cypermethrin were 0.898, 0.367 and 1.21 ppb. The carbamate insecticide, methomyl exhibited the least toxic effect against S, AM and Au strains; while the organophosphorus, malathion was the least toxic compound against W strain. Compar-ing LC₅₀ values  of the field strains with those of the laboratory strain (resistance ratio at LC₅₀ level), spinosad showed the highest RR value in AM and AU strains (78.82 and 137.25, respectively). Malathion showed the highest RR value in W strain (1744.46). Slope and RR values revealed that all tested field populations were homogenous in their response toward all tested insecticides except for spinosad. The ability to build up resistance against insecticides from different groups was discussed.