ABSTRACT Male and female D. subobscura were kept for varying periods at low temperatures (3−20°C.) and then transferred to a higher temperature (26−30°C.) and kept there until they died. It was found that during the early part of the life span, over the range 15·30°C., every day spent at a low temperature reduced the expectation of life at a higher temperature by approximately i day. Later, when the expectation of life at the higher temperature had fallen to about half its initial value, little further change in this expectation occurred with increasing age at a lower temperature. It is concluded that the life span can be divided into two phases, (i) an irreversible ‘ageing’ process whose rate is approximately independent of temperature from 15 to 30°C., but which is slower at 3°C., and (ii) a ‘dying’ process which is initiated when ageing has proceeded to a stage at which the individual is no longer capable of maintaining a steady state at the temperature at which it is living, although the same individual would be capable of maintaining a steady state at some lower temperature. The rate of the dying process is highly dependent on temperature, and it can be reversed in flies transferred to lower temperatures.