In order to determine whether it is possible to combine two or more resistance genes to the brown planthopper (BPH), allelic relationships among four resistance genes, Bph 1, bph2. Bph3 and bph4, were studied. Results indicate that bph 2, as well as Bph 1, segregates independently of both Bph 3 and bph 4, while Bph3 and bph4 as well as Bph 1 and bph2 were found to be closely linked. Therefore the combination of Bph I with Bph3, Bph I with bph4, bph 2 with Bph3 or bph 2 with bph4, respectively, in a cultivar could be achieved without any difflculty. Then, in order to search for new resistace genes, unclassified resistant cultivars were analyzed. The resistance of the cultivars, 'Andaragahawewa' and 'PTB 34' was found to be monogenically controlled by Bph I and bph2, respectively while the resistance in 'PTB 21' appeared to be controlled by the gene pair bph 2 and Bph3. Thirdly, according to the results of trisomic analysis, it was assumed that Bph3 and bph4 are located on chromosome 7.