We recently completed an electrophysiological study of the receptor neurons found in the sensilla basiconica on the maxillary palps of mosquitoes. Our results describe a class of receptor neurons whose properties could provide the afferent input required for some aspects of CO2-modulated host-locating behaviour. First, these neurons have apparent thresholds (150-300 ppm) which are at, or below, the concentration of CO2 (300-330 ppm) normally reported for ambient air. Second, their concentration-response functions are steep, such that small (50 ppm) fluctuations in concentration elicit reliable changes in activity. Third, they behave like absolute CO2 detectors in that their ability to respond to step increases in CO2 concentration is little influenced by the background concentration of CO2. And fourth, a linear extrapolation of the observed response function to the levels that might be expected near vertebrate hosts suggests that these neurons have sufficient dynamic range to cover those CO2 concentrations that should be encountered during a large portion of the behaviour likely involved in host location. The mosquito CO2 receptor neuron thus has an appropriately low threshold and a steep concentration-response function, it is not desensitized by ambient levels of stimulation, and it has a dynamic range appropriate for the distribution of CO2 concentrations expected in the environment. In addition, this sensillum contains two other receptor neurons, neither of which respond to CO2. One of these neurons responds to stimulation with very low doses of another behaviourally relevant compound, 1-octen-3-ol.