Analyses were carried out at Salisbury, Rhodesia, on the relationship between fat content and residual dry weight in newly emerged adults of Glossina morsitans morsitans Westw., following incubation at constant temperatures between 16 and 32°C for laboratory material, and between 20 and 30°C for field material. The same relationship was investigated for field-collected and laboratory-produced larvae. Sizespecific rates of fat consumption were calculated from these data, and it was shown that the rate of change at temperatures below 20°C was of a different order to that at higher temperatures. Fat reserves in teneral flies reared in the laboratory at 16–32°C were shown to be adequate in flies of 4 mg residual dry weight for survival at rest for at least 24 h after emergence, over the temperature range likely to be encountered in the field. Considerably longer periods of survival were indicated for flies of the same size emerging from field-collected puparia. The latter exhibited a marked seasonal variation in size with the largest puparia occurring at the time of lower temperatures. Smallest puparia occur in the hot-dry season, but even under these adverse conditions fat reserves of most tenerals are adequate for survival for at least 24 h. It was concluded that lack of fat was not likely to cause heavy mortality of stages passed within the puparium over the temperature range 16–32°C. After emergence, survival of smaller flies would depend on their ability to obtain a blood-meal within 24 h.