Abstract.Extracellular single cell recordings of CO2‐sensitive receptors in tsetse flies revealed a steep dose response over a range of stimuli of two to three orders of magnitude and a maximum response of approximately 70 impulses/s after exposure to a high, but naturally feasible, C02concentration of 5%. These receptor neurones are slightly sensitive to C02levels occurring in air (‐0.03%); the sensitivity to CO2above that level may be used to locate potential hosts. The C02‐sensitive neurones did not respond to some other biologically relevant odours such as octenol, butanone or p‐cresol; however, other receptor cells, some in the same sensillum, are sensitive to some of these odours.A striking feature of the C02receptors is that they appear not to adapt in their response frequencies. The spike numbers of the phasic‐tonic response remain constant in the tonic portion during continuous (or repetitively pulsed) long‐term stimulation (1 min). This unusual physiological ability would allow continuous monitoring of C02values as well as detecting potential hosts that exhale CO2. These electrophysiological results are compared to the behavioural findings in tsetse flies and to the physiological data on C02receptors of other, non‐bloodsu