The effects of the population density and food plant during the larval period on the determination of the wing-form (macropterous and brachypterous forms), and the appearance of the diapausing egg and the host plant preference have been studied with the white back planthopper, Sogata furcifera HORVÁTH. The results are summarized as follows:<br>(1) In Hiroshima Prefecture the macropterous males and females of the white back planthopper appear in the paddy fields from late June to early July for the first time. Then the brachypterous females appear and increase in number during the summer. In late August and early September the macropterous forms disappear from the paddy fields. They pass two generations during the summer in the paddy fields.<br>The wing-form of adult is determined by the environmental factors affecting during the larval period. All of the emerged adults grew the macropterous form when the larvae were reared under high population densities and fed on the weeds (Poa annua L. and Alopeculus platensis L.).<br>(2) It was found that the adult showed distinct host preference. From late June to early July, the planthoppers appear in the paddy fields, and oviposit more on the rice plant (Oryza sativa L.) than on the weed (Poa annua L.).<br>(3) The planthoppers disappear from the paddy fields from late August to early September. Just about this time, they oviposit more on the weed (Panicum crusgalli L. var. submutica MEY.) than on the rice plant.<br>(4) The planthoppers which were reared on the weed (Panicum crusgalli L. var. submutica MEY.) did not oviposit on the rice plant at all, but oviposit on the weeds such as Poa annua L. and Alopeculus platensis L.<br>(5) When the larvae were reared with leaf or sheath of the weed (Panicum crusgalli L. var. submutica MEY.) during October and November, the emerged adults oviposited the diapausing eggs. But the adults which were reared with rice seedlings or sheath of rice plant being after heading never oviposited the diapausing eggs. The diapansed eggs did not die under the low temperature of -4°C for 24 hours and overwintered in the egg stage.<br>(6) The diapaused eggs which have overwintered grow active in the middle of March and their hatching begins in the middle of April. The newly hatched larvae take the weed (Poa annua L.) for food plant and emerge late in May. This is the adult of the first generation of that year. The planthoppers which appear in the paddy fields from late June to early July are the adults of the second generation.