Absorption attributable to atmospheric IO is observed in high-resolution, high air mass solar spectra taken at the National Solar Observatory, Kitt Peak, Arizona, in March 1995. These observations, together with cross sections measured in the laboratory for the IO 2&Pgr;3/2←X2&Pgr;3/2(2,0)> rotationally resolved electronic transition, are consistent with a total stratospheric iodine mixing ratio of 0.2(+0.3-0.2) parts per trillion by volume. This result, combined with recent laboratory measurements of the rate of the reactions of IO with other halogen species, suggests that iodine chemistry is not responsible for the reductions observed in lower stratospheric ozone during the last several decades.¿ 1997 American Geophysical Union