Female Anopheles gambiae Giles mosquitoes were observed individually in a cage within a wind tunnel and their responses to pulses of carbon dioxide recorded on video tape. The range of concentrations tested revealed an 'activation' threshold concentration of carbon dioxide in the region of 0.01% above background. At this concentration, approximately 60% of the mosquitoes took off and flew upwind. Pulses of human breath, diluted with wind tunnel air to reproduce equivalent concentrations of carbon dioxide, elicited similar levels of response and the same 'activation' threshold concentration. These findings are discussed in relation to the activation of host-seeking mosquitoes.