Electron micrographs of dorsal indirect flight muscle of 37 (adult) and 68 (old) day old female houseflies were analyzed by morphometric techniques. In the muscle of the old female housefly there is no significant atrophy and the fine structure is well preserved. There is a small loss in both the myofibril and mitochondrial volumes, an increase in average myofibril size, and a 28% decrease in myofibril number. The thin/thick filament ratio and sarcomere structure are not altered. There is little decrease of particulate glycogen. The fractional volume occupied by SR at the dyads is increased. The most marked change is in the density of mitochondrial cristae which declines by 55% in old muscle and could account for the functional loss. Cristal anomalies and SR changes indicate that the membranes may be the specific target of deleterious aging effects.