The inheritance of resistance to brown planthopper, Nilaparvata lugens (Stol.), of 20 rice cultivars was studied. Single dominant genes that are allelic to Bph 3 condition the resistance in cultivars 'Ptb 19', 'Gangala' (Acc. 7733), 'Gangala' (Acc. 15207), 'Horana Mawee', 'Kuruhondarwala', 'Mudu Kiriyal' and 'Muthumanikam'. Single recessive genes that are allelic to bph 4 govern the resistance in cultivars 'Gambada Samba', 'Heenhoranamawee', 'Hotel Samba', 'Kahata Samba', 'Kalukuruwee', 'Lekam Samba', 'Senawee', 'Sulai', 'Thirissa' and 'Vellai Illankali'. The resistance in 'Ptb 33', 'Sudu Hondarwala', and 'Sinna Sivappu' is governed by one dominant and one recessive gene which segregate independently of each other. The dominant resistance genes in these cultivars appear allelic to either Bph 1 or Bph 3. Similarly, the recessive genes in these cultivars seem allelic to either bph 2 or bph 4. Further investigations are needed to conclusively determine the allelic relationships of resistance genes in 'Ptb 33', 'Sudu Hondarwala' and 'Sinna Sivappu'.