It has been hypothesized that plants contain respiratory burst oxidases which, upon activation, oxidize NADPH and generate extracellular superoxide, O2·-. These proteins are proposed to play a central role in defence against pathogens. However, plant DNA sequences that encode proteins with similarity to components of respiratory burst oxidases have not previously been reported. This paper describes the complete cDNA and genomic DNA sequence of the rice rbohA (for respiratory burst oxidase homologue) gene. The predicted RbohA product is most similar to the main catalytic subunit, gp91phox, of the respiratory burst oxidase of neutrophils. Reverse transcriptase PCR detects rbohA transcripts in both roots and shoots of healthy rice plants.