Cytochrome b558 is part of the NADPH oxidase complex of phagocytes, but it has also been proposed to function as a cellular oxygen sensor, e.g. in the carotid body. Thus, we investigated whether cytochrome b558 is present in rat primary afferent neurons. Immunohistochemistry and Western blotting using the monoclonal antibody 54.1 directed towards the large subunit of cytochrome b558, gp91phox, revealed a ubiquituous occurrence of cytochrome b558-immunoreactivity in neurons of the petrosal ganglion that innervates the carotid body, and also in dorsal root ganglia. This ubiquituous occurrence in sensory neurons of various locations and functional modalities points to a general role of cytochrome b558 in primary afferent neurons rather than involvement in a specialized function such as arterial chemoreception.