AbstractThe gills of the Atlantic cod, Gadus morhua, were studied using immunohistochemical techniques. Primary antibodies directed against serotonin (5‐hydroxytryptamine, 5‐HT) and acetylated α‐tubulin were used to visualise cells containing serotonin and nerve fibres, respectively. Three morphologically different 5‐HT immunoreactive cell types were distinguished: (I) Neuroepithelial cells (NECs), which were abundant along the distal half of the efferent filamental arteries (EFAs), and particularly formed distinct clusters at the individual filamental tips, (II) bipolar neurones running next to the EFAs and (III) multipolar neurones innervating the proximal parts of the EFA. In addition, the study revealed a well‐developed system of nerve fibres, some of which form plexuses in association with the NECs. A relatively rich innervation of the proximal part of the EFAs, in conjunction with the EFA sphincters was also observed. Delicate varicose terminals surround the bases of the efferent lamellar arterioles. The localisation of distinct clusters of NECs at the individual filamental tips and the close connection with nerve terminals suggests a function as external branchial oxygen receptors.