1. The flight activity patterns of groups and individuals of sugar-fed A. taeniorhynchus females have been studied under different light regimes, by recording of flight sound as an indicator of flight activity.

2. In an LD 12:12 regime, flight activity occurs both at light-off and light-on, forming a bimodal ‘alternans’ pattern. This basic pattern of flight activity persists with a periodicity of 23.5 h under continuous DD, but under continuous LL is masked over by irregular excessive outbursts of activity.

3. The flight activity rhythm originates for the first time in the adult stage, and it is not carried over from rhythms of developmental stages.

4. Flight activity rhythms can be entrained to a new light regime within 24-36 h which is rather fast. An early light-off does not reset the phase of the rhythm, but a delayed light-off does.

5. The flight activity rhythm can be entrained to 24 h light regimes other than LD 12:12. But a single stimulus of less than 12 h is not effective in initiating the bimodal circadian rhythm.

6. Frequency demultiplication within certain limits can entrain the flight activity rhythm to 24 h.

7. This flight activity rhythm of A. taeniorhynchus is compared with the activity rhythms of Aopheles gambiae and Culex pipiens pallens.

8. It is concluded that the basic bimodal alternans pattern of flight activity is a persistent property of the circadian oscillating system, which suggests that other activity rhythms involving flight are dependent on the same rhythm.

Contribution no. 260, Florida Division of Health, Entomological Research Centre, Vero Beach, Florida 32960. This work was supported in part by Grant no. AI-06587, National Institutes of Health, U.S. Public Health Service.