Fluctuations in the wing-form ratio of the brown planthopper (BPH), Nilaparvata lugens, were examined by collecting fifth-instar nymphs from rice fields. Two peaks of brachypterous females were observed at 40–50 days after transplanting (DAT) and 75–85 DAT (5 days before heading–10 days after heading). After 75–85 DAT, the percentage of brachypterous females decreased with an increase in BPH density. However, the BPH density was still low after 40–50 DAT when the first decrease in the percentage of brachypterous females occurred. Thus, not only population density of BPH, but also the rice plant stage seems to affect the fluctuations in female wing-form ratio. The SPAD readings of rice leaf, which are associated with leaf nitrogen content, fluctuated with rice plant stage and showed two peaks at 30–50 DAT and 65–80 DAT. The fluctuation pattern of the SPAD readings was similar to that of the percentage of brachypterous females. This suggests that the percentage of brachypterous females fluctuated in accordance with a decrease in nitrogen content of rice plants.