Rapid generation of O 2 2 and H 2 O 2 , which is reminiscent of the oxidative burst in neutrophils, is a central component of the resistance response of plants to pathogen challenge. Here, we report that the Arabidopsis rbohA ( for respiratory burst oxidase homolog A ) gene encodes a putative 108-kD protein, with a C-terminal region that shows pronounced similarity to the 69-kD apoprotein of the gp91 phox subunit of the neutrophil respiratory burst NADPH oxidase. The RbohA protein has a large hydrophilic N-terminal domain that is not present in gp91 phox . This domain contains two Ca 2 1 binding EF hand motifs and has extended similarity to the human RanGTPase-activating protein 1. rbohA , which is a member of a divergent gene family, generates transcripts of 3.6 and 4.0 kb that differ only in their polyadenylation sites. rbohA transcripts are most abundant in roots, with weaker expression in aerial organs and seedlings. Antibodies raised against a peptide near the RbohA C terminus detected a 105-kD protein that, unlike gp91 phox , does not appear to be highly glycosylated. Cell fractionation, two-phase partitioning, and detergent extraction indicate that RbohA is an intrinsic plasma membrane protein. We propose that plants have a plasma membrane enzyme similar to the neutrophil NADPH oxidase but with novel potential regulatory mechanisms for Ca 2 1 and G protein stimulation of O 2 2 and H 2 O 2 production at the cell surface.