The fish literature is peppered with evidence of two types of putative oxygen chemoreceptors: internal receptors that sense changes in blood PO2 and external receptors that sense changes in water PO2. The exact location and morphology of these cells remains unknown. Our goal was to identify these elusive chemoreceptors using a facultative air breather, the bowfin. One group of fish were exposed to prolonged hypoxia (42 torr for 7 days) and had access to the surface of the water to breathe (hypoxic but not hypoxaemic). Another group of fish were exposed to the same hypoxic protocol but did not have access to the surface to breathe (hypoxic and hypoxaemic). Following exposure the fish were sacrificed and their gills sampled. We identified 3 types of putative oxygen chemoreceptors. Type I cells were found near the efferent filament artery, had no cellular projections and were immunopositive for serotonin (5‐HT), acetylcholine (ACh), synaptic vesicle (SV2) markers and were innervated (zn‐12). Type II cells were bipolar neurons found in the central venous sinus (cvs) and immunopositive for 5‐HT, ACh, SV2 and zn‐12. Type III cells were also found in the cvs, had no cellular projections and were immunopositive for 5‐HT but not for SV2 or ACh and were rarely innervated. Type I cells were significantly larger in bowfin without access to air than in bowfin with access to air, suggesting that these cells are internal chemoreceptors.