Four colonies of Lycoriella mali (Fitch) collected from different mushroom farms in southeastern Pennsylvania and Delaware were examined for resistance to topically applied permethrin in laboratory studies. Low to moderate (8- to 47-fold at LD50) resistance was discovered in all four colonies compared with a reference strain not previously exposed to permethrin. Resistance at the LD95 was from 34- to 67-fold. The LC95 of three of the colonies was approximately equal to the field rate for permethrin, whereas one colony’s LC95 was below the field rate. Slopes of the dose-mortality lines of the four colonies ranged from 0.82 to 1.31, suggesting a potential for higher levels of resistance in these populations with continued selection pressure. The large concentration of mushroom farms in the area, short generation time of L. mali , numerous applications of permethrin, and history of pesticide use are likely to have influenced the development of resistance to permethrin in this species.