Reactive oxygen species (ROS) are proposed to function as diffusible signaling molecules in plant immune response. Rice respiratory burst oxidase homologs (Osrboh genes) are proposed to play a role in ROS generation. We examined a role in rice immune responses of four Osrboh homologs, OsrbohA, OsrbohB, OsrbohD, and OsrbohE. OsrbohA and OsrbohD transcripts were induced after inoculation with an incompatible N1141 strain of Acidovorax avenae, whereas OsrbohaB and OsrbohE mRNA levels did not obviously change even after inoculation with the incompatible strain. We examined the function of the Osrboh genes in ROS generation and in the plant immune response using RNAi-based knockdown in rice cells. OsrbohA and OsrbohE knockdown lines showed that rapid H₂O₂ generation is caused by OsrbohA, whereas OsrbohE is involved in late H₂O₂ production during the immune response. Hypersensitive cell death was decreased only in the OsrbohA knockdown line. We further demonstrated that among immune related genes, the induction of EL2 and LOX genes is controlled by ROS generated by OsrbohE, whereas expression of Cht-1 gene is regulated by both OsrbohA and OsrbohE. These results indicate that the ROS molecules generated by OsrbohA and OsrbohE regulate different signaling pathways in the plant immune response.