The sodium channel is the primary target of pyrethroid insecticides. Molecular studies on houseflies and fruitflies have provided evidence that modified sodium channels can contribute to pyrethroid resistance. Physiological and genetic studies have also suggested that a modified sodium channel may be responsible for pyrethroid resistance in tobacco budworm. To test this hypothesis directly, we have cloned a sodium channel gene from tobacco budworm. We present evidence that molecular variation at this locus is linked to pyrethroid resistance in a laboratory strain. We have developed a rapid method for screening for genetic variation at the sodium channel locus. We describe how this method might be integrated into existing, ongoing monitoring efforts, to more precisely delineate the occurrence of this specific resistance mechanism in the field.