There was a wide range of resistance levels to organophosphates in more than 30 populations of diamondback moth (Plutella xylostella) collected from various locations throughout Japan. Based on their resistance levels and spectra, those populations were divided into five groups. The resistance ratio in thiono-type insecticides was higher than that in phosphate- and dithio-types. High resistance to organophosphates was found in the areas where crucifers had been grown on a large scale throughout the year under frequent applications of insecticides. Organophosphate resistance could be readily build up by selecting under the use of organophosphates. However, the resistance was not stable. High resistance to pyrethroids has been observed in various places of southwest Japan since 1984 which was only a few years after the introduction of insecticides pyrethroid. The pyrethroid resistance of the moth, indicating cross-resistance to a number of types of pyrethroids, was more stable than the organophosphate resistance. Decrease in susceptibility to tertiary amine cartap, BT and IGR is still moderate.