Population dynamics of the brown planthopper (BPH), Nilaparvata lugens Stal, was investigated at nine study sites in the irrigated coastal lowland of West Java, Indonesia, where rice was cultivated under intensive modern agricultural practices. The BPH populations in this region were definitely characterized by the low initial immigrants in a year, followed by the subsequent high population growth. In the wet cropping season in particular, populations multiplied about 2,000 times in size in the period from initial to 2nd, or peak, generation, reaching quite often the destructive level despite their low initial densities. In the dry cropping season, however, the levels of the population growth rate and the peak population densities were much lower than those in the wet crops. The abundance of natural enemies such as arthropod predators and parasitic wasps played a major role in determining the population level in the peak generation in the two cropping seasons. The densities in the peak generation were predictable with fairly high accuracy on the basis of the densities of the initial or previous seasonal generation in the wet crops. In the dry crops, in contrast, the population growth rate widely varied depending on the condition of water-supply to each paddy field, which exerted a major influence on the BPH population fluctuations among the fields.