The present experiment was carried out in order to investigate the relationship between the life-cycle of green rice leafhopper and rice yellow dwarf occurring in a large scale.<br>The results obtained are summarized as follows;<br>1. The adult of the overwintering generation begins to emerge out from the 2nd decade of February to the 1st decade of March, and almost all of the imagoes become adults in the beginning of April. The population of these adults decreased rapidly from the middle of May, but some individuals survived until the end of May.<br>2. The population density in spring counted by sweeping did not coincide with the number of individuals caught in light trap. First and 2nd decades of April when the population of imagoes become to minimum seems to the end of emergence.<br>3. As the result of rearing test from winter to mid-summer, imagoes appear till the 1st decade of April in the first generation, and start to emerge from the 2nd decade of May and survive till the 1st decade of July. The second generation adults appear from the beginning of July till middle of August, and the third generation continues.<br>4. The relation between the life-cycle of leafhopper and the growth of rice plant is that the damage from overwintering and the first generations is heavy in case of early cultivated rice plant, and the second generation adults increase in number at harvesting time. Rice seed of normal cultivation will be sown about at the end of overintering adult stage, and that of late cultivation at the end of first generation in adult stage.<br>5. Diseased plants are recognized at the end of June about 70 days after virus is transmitted into early cultivated rice plant by overwintering hoppers. The latent period of the virus in the hopper taken from diseased plant was from 21 to 28 days. These hoppers, therefore, will become able to transmit virus about at the end of July.<br>6. Some adults at the beginning of April and at the end of July are recognized as virus vectors, but imagoes from the middle to end of May and adults from the middle of June to the middle of July, i.e., leafhoppers of the first generation and early period of the second generation have no virus.<br>7. Infection period of yellow dwarf virus to the early cultivated rice plant continues from the middle of May. However, it becomes non-infectious until the end of July. No symptom of yellow dwarf is observed on the early cultivated rice plant infected twice while it is growing, appears on young shoot from the rice stubble.<br>8. The wild plant, Alopeculus auqualis SOBOL was examined in spring but was found not infected.<br>9. The overwintering generation of leafhopper is most important as the vector of the yellow dwarf virus as the primary infection in the paddy field. It is considered that the generations from the 2nd are chiefly transmitting virus from these diseased early rice plants.