1. The pattern of non-specific flight activity in Anopheles gambiae females appears to be bimodal, with an initial peak followed by about 12 h of secondary activity, with a maximum 6-10 h after the main peak.
2. The secondary activity is more apparent when the activity level is high, and can account for over 60% of the total activity. When D < 12 h activity is curtailed by the inhibitory effect of light, but there may be a minor peak in the light period, 8-9 h after light-off.
3. The main peak normally follows light-off, but is delayed in LD 3:21 and 1:23.
4. There may also be a peak at light-on if it is abrupt and occurs during the active half of the cycle. The peak is not observed if the light-intensity is changed slowly.