The experiments reported in this thesis examined the effects of laboratory thermal selection on a range of characters in two sets of three replicate lines of Drosophila m elanogaster , maintained at 16.5°C and 25°C for over four years. The duration of developmental stages in males and females from both selection regimes was greater when reared at 16.5°C than when reared at 25°C. An evolutionary response of development times to temperature was also evident. Times to pupariation of 25°C selected larvae were longer than those of 16.5°C selected larvae, when reared at 16.5°C or 25°C. Times to adult eclosion were longer for 25°C selected lines compared to 16.5°C selected lines, when both were reared at 16.5°C, but the situation was reversed at 25°C. Pre-adult survival was higher in selected lines when they were reared at their own selection temperature. This is clear evidence of rapid thermal evolution and adaptation in the selected lines. Larval growth patterns were also examined. Larvae from both selection regimes reached a higher weight, and took almost twice as long to develop, when reared at 16.5°C rather than 25°C. At both growth temperatures, 16.5°C selected larvae had higher mean weights at almost all sampling intervals in comparison to 25°C selected larvae. Starvation experiments showed that larvae from the thermally selected lines had higher critical weights for pupariation when reared at the temperature at which they had been selected compared to larvae from the other selection regime reared at the same temperature. Results suggested that 16.5°C selected lines may be allocating m ore nutrients to growth as opposed to somatic maintenance. Larvae from thermally selected lines showed evidence of adaptation, with respect to larval competitive ability. Greater competitive ability was shown by larvae from each selection regime when they were reared and tested at the temperature at which they had evolved, compared to larvae from the other selection regime, reared and tested at the same temperature. Development and evolution at 16.5°C both resulted in an increase in thorax length and wing area. The developmental and evolutionary responses of increased wing area were found to be due almost entirely to an increase in cell size rather than number. The similarity between the developmental and evolutionary responses of both body size and cell size was suggestive of adaptive phenotypic plasticity. The life spans of both sexes and the fecundity and fertility of the females from both selection regimes were measured at both experimental temperatures (16.5°C &25°C). Adult longevity was much greater when flies were reared and maintained at the lower temperature. The effect of selection temperature depended upon the temperature at which longevity was measured. Flies from both selection regimes showed signs of adaptation, having greater longevity when tested at the experimental temperature at which they had evolved, compared to flies from the other selection regime tested at the same temperature. Thermal selection had a significant effect upon fecundity. This character showed d ear evidence of adaptation. Females from each selection regime showing greater fecundity at almost every sampling interval, when reared and tested at the temperature at which they had evolved, compared to females from the other selection regime tested at the same temperature. The results suggest that there may be a trade-off between longevity and fecundity at the two temperatures. Alternatively, adaptation to one temperature may imply loss of adaptation to the other which the flies no longer encounter.