The orientation of Aedes aegypti in an upward or downward vertical air current was studied. The vertical movements of A. aegypti which have been activated by CO2 are always against the air flow and toward the inlet end, where the insects alight and probe. With sufficient illumination A. aegypti locate an invisible artificial host whether it is at the top with air moving down or at the bottom with air moving up. They do not attempt to locate a visible, black, artificial host placed downstream. Body attitude is similar in upward or downward air flow. The mosquitoes follow the vertical movement of a surrounding spiral stripe pattern, whether stripe movement is upward or downward, whether air is still or moving vertically with or against the stripes. They will not fly against stripe movement to reach an upstream host, real or artificial. Repellent vapor destroys the upstream orientation when air flow is vertical but not when the flow is horizontal. The stripe-following action is not affected by repellent. The experiments indicate that A. aegypti possesses a vertical-air-speed sensor, of a mechanical type, to direct movement up or down in a vertical air stream. The cybernetic system must be complex and requires cooperative use of mechanical and optical sensory information channels. The initial action of repellent vapor appears to be peripheral rather than central to the nervous system, and to affect only the mechano-, chemo-, and thermo-receptors.