Susceptibilities of three field strains of the diamondback moth, Plutella xylostella L., against dichlorvos, prothiophos, phenthoate and fenvalerate were determined, and were compared with those of a susceptible strain. Insecticide solutions were sprayed to third instar larvae with a rotary spray tower. Susceptibilities of three field strains (Aichi, Hyogo and Okinawa) to dichlorvos, prothiophos and phenthoate were lower than those of the susceptible strain. Among three field strains tested here, no resistance to fenvalerate was found.