1. The circadian cycle of flight activity of individual, sugar-fed, Anopheles gambiae females has been studied, using the flight-sound as an indicator of activity.
2. In an LD 12:12 régime (alternating 12 hr. light: 12 hr. dark), activity peaks follow both light-off and light-on. The mosquitoes are moderately active in the dark, but are inactive in the light after the first half-hour.
3. If the light period is extended, the activity is delayed until light-off. Light appears to have an inhibitory effect, but the insects may show some activity towards the end of a 48 hr. light period.
4. Cyclical activity continues in constant dark with a period of approximately 23 hr. A late light-off resets the cycle; an early light-off does not.
5. When the LD 12:12 régime is advanced by 6 hr. the cycle is re-entrained within 2-3 days. Light in the second half of the subjective night appears to advance the cycle.
6. It is suggested that the activity is controlled by an endogenous rhythm which possibly controls the release of an excitatory hormone. The effect of light may be mediated through an inhibitory hormone.