A series of field observations and rearing experiments were carrried out in Florida to gain information on the duration of the larval stage and the time of day of pupation and emergence of the salt-marsh mosquito, Aëdes taeniorhynchus (Wied.).The technique adopted in a series of experiments is described.It was found that the duration of the larvel stage is dependent on several factors of which water temperature is probably the main factor. Hunger was also shown to be important either through lack of food or overcrowding. Larvae which later became male adults pupated before those which became female imagines.The period of maximum pupation is determined by the distribution of light and darkness, 83 per cent. of all pupation taking place during the evening periods, 15 to 24 hours after experimental midnight, and 48 per cent. in the period just after sunset, 18 to 21 after experimental midnight. The tendency to pupate around sunset is thought to have just as much accelerating as retarding effect on the larval development.Temperature is considered to be the only factor of importance in determining the duration of the pupal stage which varies from 61 hours at 20·8°C. to 45 hours at 25·5° and 37 hours at 29°.Both in nature as well as under controlled conditions in the laboratory it was found that, for a large number of larvae hatched simultaneously, the emergence of adults usually lasts three or fours days, with a maximum at the same hour on each of these days. The period of emergence is proportional to the duration of the post-embryonic development and may under unfavourable conditions last weeks.Maximum emergence of imagines has been found to occur at various times of the day. This is as should be expected since the hour of pupation is fixed to a certain period of the day, and the duration of the pupal stage is solely dependent on the temperature.The difference in the duration of pupation in males and females is very small but owing to the rather marked sexual difference in the duration of the larval stage, 129 hours for males against 145 for females at 29°, there is a preponderance of males during the first emergence maxima and of females in the last.It is concluded that if the temperature of the water in which the larvae are living in the field is known, it should be possible to predict at what hour of the day the emergence maxima will take place.