The effect of air damping on the resonant frequencies of forced vibration of stretched filaments is studied by observing the shift in frequency when the surrounding medium is changed from air at atmospheric pressure to hydrogen at the same pressure, or to air at reduced pressures. For fibers of linear density less than about 10 micrograms per centimeter, the shift between atmospheric air and vacuum exceeds one cycle per second, becoming greater as the fibers become finer. The frequency shifts observed experimentally are compared with those predicted by certain formulas of Stokes. It is found that the theory works well enough to allow the effect of air damping to be taken into account in measurements of linear density by means of the vibroscope. However, the observed shift seems to be about one or two cycles per second higher than that calculated in the case of small shifts, and about 20 or 30 percent higher in the case of large shifts. A rigorous evaluation of the applicability of Stokes' theory would require further investigation.