Abundance of the yeast-like symbiote (YLS) in brown planthopper (BPH), Nilaparvata lugens Stal, collected from rice fields in China and obtained from International Rice Research Institute (IRRI), was monitored. The role of YLS in nymph survival rate, development and adult reproduction, and activities of transaminases in BPH on different resistant rice varieties were examined. Also, the relationship between YLS and virulence shifts of BPH to resistant rice varieties was discussed. The results indicated that the virulence of BPH populations to resistant rice varieties was strongly related to the abundance of YLS in 5th instar nymphs and brachypterous females of BPH field populations at Nanning in South China and Hangzhou m East China, showing the more abundance of YLS in BPH, the higher virulence of BPH to resistant rice varieties, while no obvious differences in YLS abundance were monitored between three BPH biotypes. Numbers of YLS reduced significantly in female adult of BPH biotype 1 fed on resistant rice varieties IR26, Mudgo and ASD7, compared with that fed on susceptible rice variety TN1. The lowest YLS abundance of BPH from field population was found in 2nd generation successively fed on resistant varieties, and the number of YLS was resumed at 4th generation when the BPH has adapted the resistant varieties. The markedly lower nymphal survival rate, adult fecundity and activities of alanine and aspartic transminase, and significantly longer nymph duration in the three aposymbiotic biotypes than in the symbiotic ones were recorded on both susceptible and resistant rice varieties. However, the much greater differences between aposymbiotic and symbiotic BPH were found on the inadaptable resistant rice varieties than that on the fitness resistant rice varieties. Those results implied that YLS in BPH should playa crucial role in the virulent shift of BPH populations to resistant rice varieties.