The brown planthopper Nilaparvata lugens (Stl) (BPH), a major pest of rice, has produced successive pest outbreaks in China in recent years, resulting in serious damage to rice production. As Guangxi Municipality is the principal area of China where immigrants first arrive, a better understanding of the dynamics and the source areas of immigration of populations of the pest in early rice crops in Guangxi will be beneficial for the prediction and suppression of this pest in other rice cropping areas of China. In this paper, the population dynamics and characteristics of the pest in double-cropped early-season rice fields in Nanning were studied through systematic investigation of female ovarian maturity. The spatio-temporal distributions of source areas of BPH immigration peaks in Guangxi in 2007 were analyzed by using HYSPLIT, a trajectory analysis software for the simulation of migration trajectories, and light trap catches of BPH. We obtained the following results: (1) The population density of BPH in the late variety rice crop was significantly higher during the late period than in that of the early variety crop, and the period of occurrence and crop damage were also longer in the late variety, which might provide an adequate source for population outbreaks in northern China later in the season. (2) The population characteristics of BPH for each generation in double-cropped early-season rice fields in Nanning showed that the mated ratio and proportion of matured ovaries (Ⅲ-Ⅴgrade) in the 2nd generation (21April-20May) were 80.9% and more than 70% respectively, so the 2nd generation was estimated to be mostly immigrants, but supplemented by some local breeding. However, the mated ratio and proportion of matured ovaries (Ⅲ-Ⅴgrade) at the beginning of the 3rd generation (21May- 6June) were 60.2% and 72.2% respectively, falling to 19.4% and 25% later (7June-20June). Therefore, the 3rd generation resulted mostly from local emergence, but with some immigration during 21May- 6June and some emigration during 7June-20June respectively. The mated ratio and the proportion of immature ovaries (Ⅰgrade) in the 4th generation were 17.7% and 80.7% respectively, indicating that these were largely emigrants. (3) Simulations of migration trajectories during the major immigration peaks in 2007 indicated that most of the immigrants came from the centre of Vietnam or Laos (18-20°N，102-106°E) from late April to early May; from the north of Vietnam or Laos (20°- 22°N，104-107°E), and also from Hainan Province and Leizhou Peninsula during mid May to late May; while the massive immigration to northern Guangxi Municipality in early June probably arrived from the north-east of Vietnam (21-22°N，106-108°E) and the south-west of Municipality. The analysis of weather conditions during migration peaks showed that strong low level jets carried large-scale invasions of BPH into Guangxi, while heavy rainfall caused mass descent into the immigration areas.