To study the effects of environmental factors on the flight activity of certain livestock insects, particularly horn flies, Haematobia irritans (L.), and stable flies, Stomoxys calcitrans (L.) (Diptera: Muscidae), an aktograph was constructed that can detect, amplify, and record electrical changes on an electrode caused by nearby insect flights. The flight activity of insects held in 2 identical environmental chambers can be continuously monitored. Results of these preliminary studies using stable flies indicated that there was a higher level of activity in light than in dark for both sexes; males were more active than fema'es under all light conditions; there was a difference in the pattern of activity in light as compared with dark and for males as compared with females; and a peak of activity occurred at about 16–18 hr postfeeding followed by a gradual decrease, probably resulting from debilitation of the flies.