The population growth of the brown planthopper in paddy fields and the infestation with rice ragged stunt disease were studied during a three-year period since 1979 in the Central Plain, Thailand. The population trends of the insect were also observed in Northwest Java, Indonesia during the 1982/83 wet season. The immigrants of the brown planthopper produced two generations in a discrete way until harvest of rice. The females showed brachypterous wing forms with a ratio of more than 90% in the first generation. This pattern was commonly observed both in Thailand and Indonesia. The immigrants during the early weeks after transplanting of rice played a significant role in the infestation with the disease. These early immigrants did not establish their field populations on rice. The infection occurred within a 40-day period after transplanting. Secondary transmission in the later period did not cause any disease symptoms until harvest. The infection during the nursery period was important only under severe epidemic conditions.