Age-associated changes in the flight muscles of higher insects, using male housefly as an example, have been discussed. The duration of sustained flight activity declines with age. During the first trimester of adult life, the mitochondrial volume in the flight muscles increases by additional biosynthesis. Individual mitochondria become further enlarged by the fusion of adjacent organelles. In old flies, a small proportion of mitochondria show degenerative changes. In senile flies, focal areas of the flight muscles undergo cytolytic changes. A variety of metabolic changes involving enzyme activities, protein synthesis, substrate levels and bioenergetics have been reported in flight muscles of aging insects. The mechanism of the loss in flight capacity of old insects, however, remains unknown.