The objective of this investigation was to determine if actomyosin ATPase activity in flight muscles is correlated with life expectancy of houseflies. All houseflies lose flying ability before death which permits the identification of shorter-lived flightless 'crawlers' from their longer-lived cohorts, the 'fliers'. Life expectancy of crawlers is about one-third shorter than that of the fliers. Flying performance of houseflies, as measured by the total duration of flying activity during 1 h periods, average duration of flights and the number of rest stops, was highest at 4 days of age and declined thereafter. Actomyosin ATPase activity was higher in the fliers than in the crawlers of the same age. Abolition of flight, by surgical removal of wings at 1 day of age, had no effect on the enzyme activity. Results are interpreted to suggest that actomyosin ATPase activity is correlated with physiological rather than chronological age of flies.