The isotope ratio of terrestrial air helium is believed to be constant on a global scale since the mixing time for the atmosphere is significantly shorter than the residence time for helium. Recently, the 3He4He ratio at Ueno Park, central Tokyo, Japan, was found to be significantly lower than previously measured values. We report here a change in the atmospheric helium isotope ratio from 1.362¿10-6 in December 1977 to 1.339¿10-6 in September 1988 or a decrease of 1¿10-9/year. Although this change could be due to local/nonglobal effects, minor changes in the atmospheric flux balance for helium or experimental artifacts, the observations are also consistent with a significant influx of a low 3He/4He ratio source to the atmosphere. The magnitude of the flux, 0.48-2.9¿1016 cm3STP He/year, is compatible with estimates of the anthropogenic release of crustal helium from gas and oil production from the solid Earth. Because of the inert chemistry of helium, a quantification of this change may provide a marker against which to calibrate the absolute flux and retention of anthropogenic CO2 in the atmosphere. ¿ American Geophysical Union 1989