Mating-choice and host preference tests were carried out in the rice (Oryza sativa)-feeding (RF) and water-oat (Zizania latifolia)-feeding (WOF) types of Chilo suppressalis W. The mating-choice tests revealed that matings occurred not only with in the same type, but also between the two types. However, the peaks of mating-time of females were different, i.e., that of the RF type was 3 hr earlier than that of the WOF type. The host preference tests revealed that females of both types have a tendency to select their own host for oviposition.