The modulation of tropospheric ozone by a propagating gravity wave is inferred from the analysis of more than 200 lidar profiles obtained over a 14-hour period during September 3-4, 1993, near Rollinsville, Colorado (39.9¿N, 105.3 ¿W, 2.68 km msl). Periodic fluctuations of up to 30% in the ozone mixing ratios are attributed to a nearly monochromatic gravity wave with a downward phase velocity of -0.08¿0.01 m s-1, a mean vertical wavelength of 3.0¿0.5 km, and an apparent frequency of (1.7¿0.4)¿10-4 s-1. Wind profiles from nearby radar soundings suggest that these fluctuations are induced by a gravity wave that propagates from the NW to SE with a horizontal wavelength of ~180 km. These results imply that gravity waves may contribute significantly to the short-term (≤1 day) variability of ozone and other trace gases in the troposphere. ¿ American Geophysical Union 1996