We studied the genetic architecture of host plant adaptation in two populations of brown planthopper Nilaparvata lugens (Homoptera: Delphacidae): one feeding on cultivated rice Oryza sativa and the other feeding on a weed grass Leersia hexandra. Proportional weight change, survival and development time of inbred Leersia- and rice-feeding lines, F1, F2, and backcross classes have been examined. Most of the performance differences among populations seem to be controlled by a few genes. Dominance of rice population alleles over Leersia population alleles was quite strong, and there was evidence for epistatic interaction. Nymph-to-adult survival of the cross classes was found to be highly correlated with mean weight change of the adults. These results are discussed in relation to a presumed host shift from Leersia to rice.