The amplitude of wingbeat of Drosophila virilis Sturtevant in moist air at a temperature of 27 ± 1 degrees C. and at a series of gas pressures ranging from 3863 mm. Hg to 95 mm. Hg, was recorded photographically. The vertical amplitude of the wingbeat decreased progressively with increase in air pressure. The excursion of the wing was curtailed in the same proportion at both extremes of the stroke. These changes are attributed to alterations in atmospheric density which resulted from the change in gas pressure. It is calculated that amplitude varies approximately as p-0.09, No significant change was observed in the limits of angular excursion of the wing in a fore and aft direction, although the envelope of the wing movement, as seen in lateral view, broadened as pressure was reduced. It is shown that the measured changes in amplitude and frequency compensate only in part for the change in load which results from alterations in air density. The possible role of power losses due to wing inertia is discussed, and it is concluded that these represent only a minor factor in the observations. Arguments are presented which suggest that alterations in gas density provoke compensatory changes not only in wingbeat frequency and stroke amplitude, but also in the angle at which the wings attack the oncoming stream of air. The degree of compensation required would be provided by doubling the angle of attack at 95 mm. Hg and halving it at 3863 mm. Hg.