The changes in insecticide susceptibility of the diamondback moth (Plutella xylostella L.) collected from cabbage fields in Iwaoka, Hyogo Prefecture, Japan were investigated during the period 1985 to 1990. The susceptibility of the diamondback moth to acephate had been continuously low since the initiation of the experiment conducted in 1985. The susceptibility to cartap and BT was high in 1985 when they were both introduced to Iwaoka for the first time. However, since 1988, their effectiveness has gradually declined, followed by a drastic decrease since 1989. Susceptibility to phenthoate has rapidly receded due most probably to intensive sprays repeated every year. The combination of fenvalerate with malathion applied for 2 years, resulted in the rapid decline in effectiveness, starting in June 1986. However, its effectiveness slowly recovered in 3 years after its use was discontinued. It is very likely that the development of insecticide resistance in insects is heavily dependent on the spray frequencies in fields. Effectiveness of insecticides for several moth populations in Hyogo Pref. was examined. The populations in the highlands of Hidaka and Kinosaki showed high susceptibility to all the insecticides tested. On the other hand, the populations in the southern lowlands of Hyogo, such as Mihara and Hirano, possessed very low susceptibility to them at the initial stage of application. Their susceptibility level also declined year by year, however. Dimethylvinphos and chlorfluazuron are most probably recommendable for practical use in fields.