PREVIOUS work on the initiation of the pupation and flight activity rhythms in mosquitoes1–3, has been interpreted4 as showing that there is an ontogenetic distinction between the ‘clocks’ controlling development and adult behaviour. Results with Aedes taeniorhynchus1,2 suggest that the developmental clock stops after controlling the time of pupation and that the adult behavioural clock does not start until some time after emergence. In Culex pipiens pallens3 the adult clock seems to start just before adult emergence. By contrast, the results reported here indicate that in the mosquito Anopheles (Cellia) gambiae Giles, the flight activity rhythm can be entrained during development at least as early as the second larval stage. The same Zeitgeber also sets the phase of the pupation rhythm.