The role of glycogen in the flight physiology was studied for 2 spp. of flies, Drosophila junebris and Lucilia sericata. Glycogen was detd. by microchemical methods. The flight ability was measured atroboscopically in terms of the total number of wing-beats, under standardized conditions, in continuous flights to exhaustion. Glycogen was found to be of primary importance in the physiology of flight. During continuous flight the conc, of this substance gradually decreases in both the entire animal and the thorax. The decrease in glycogen during the first stages of such flights has no- marked effects on the intensity of flight, in terms of the frequency of wing-beat. Near the end of continuous flight the conc, of glycogen becomes limiting and wing-beat frequency rapidly decreases until flight ceases before the frequency becomes as low as 100 double-beats per second. Both the flight ability of Drosophila and the conc, of glycogen vary as functions of age. During the first wk. of adult life the avg. length of flight increases from 26 min. on the first day to 110 on the 7th and the total number of wing-beats from 225,000 to more than a million. Simultaneously the glycogen conc, rises from about 2.5-6% of the live wt. In animals older than 2 wks. the flight ability and glycogen conc, decrease rapidly and then more slowly until, by the 33d day, ayg. length of flight is reduced to 19 min. (170,000 double wing-beats) and the glycogen conc, to about 3.5% of the live wt. This correlation, although not exact, suggests that the physiol. ageing of the flight ability results to a large degree from the simultaneous changes in the conc, of glycogen.