In experiments with Aëdes aegypti (L.), fourth-stage larvae bred at 30°C. survived exposure to 0·5°C. for 7 hours, but none recovered after exposure for 17 hours, although nearly all those from the same culture that had been acclimatized for one hour to 18 or 20°C. did so. No such effect followed acclimatization at 10°C., at which larvae are in chill coma. Larvae acclimatized at 18°C. survived exposure to 5°C. for longer than did those from 30°C. Earlier work showed that the thermal death point, for a one-hour exposure, was 42°C. for larvae acclimatized at 30°C. but rose to 44°C. for larvae acclimatized to 37°C. It is concluded that the effects of high or low extremes of temperature may be substantially modified by a brief period of acclimatization at a less extreme temperature.