Five field-collected populations (TR-CP, UR-CP, JR-CP, GR-CP, and BR-CP colonies) of northern house mosquito, Culex pipiens pallens, identified by polymer chain reaction (PCR) were tested for susceptibility to 10 different insecticides by direct-contact mortality bioassay in comparison with an insecticide-susceptible KS-CP strain. Marked regional variations of insecticide susceptibility were observed. Extremely high to high levels of resistance were obtained: bifenthrin, resistance ratio (RR) = 1–520; β-cyfluthrin, RR = 16–397; α -cypermethrin, RR = 9–343; deltamethrin, RR = 1–40; etofenprox, RR = 2–42; permethrin, RR = 2–12; chlorpyrifos, RR = 2–675; fenitrothion, RR = 0.5–364; and fenthion, RR = 2–360. All of the colonies were susceptible to one or more of the insecticides examined. These results indicate that careful selection and rotational use of these insecticides should result in continued satisfactory control against field populations of northern house mosquito.