The release of an insect pest-resistant variety and subsequent adaptation of the insect species to that variety through the process of biotype selection has occurred on many cultivated crops. The present study was designed to obtain detailed information on some of the processes involved in the selection of biotypes ofNilaparvata lugens(Stål) on two rice varieties with different major genes for resistance. TheN. Lugenspopulation selected for the study, which had originally been collected in the field and reared on a susceptible variety for 12 years (ca. 140 generations), initially had a low survival on the resistant rice varieties Mudgo (Bph 1 gene) and ASD7 (bph 2 gene) and high survival on the susceptible variety, Taichung Native 1 (TN1). Monitoring the survival and length of the developmental period for seven generations indicated a progressive increase in the survival and shortening of the nymphal period in each generation on the heretofore resistant varieties until they were similar to those on TN1. The shift in the population to a more virulent biotype was also confirmed by the seedling bulk test. The selected populations reared on Mudgo or ASD7 were not able to kill Mudgo or ASD7, respectively, in the fifth generation but by the eighth generation were able to do so. In addition, adult longevity and fecundity on the resistant varieties increased during the selection process.