The gravity wave/quasi two-day wave interaction is an example of gravity wave/large-scale flow interaction, a subject of great interest but insufficient understanding. Here we present evidence, from MF radar data, for the quasi two-day wave modulation of gravity wave activity in the summer mesosphere over Urbana, Illinois (40¿ 10' 10″ N, 88¿ 09' 36″ W). A composite analysis of two-day wave events, identified using the meridional winds at 84 km, is performed on the four-hour mean winds and four-hour standard deviations about those means, the latter being representative of gravity wave activity. Our results show that quasi two-day variations in the winds modulate gravity wave activity at higher altitudes. ¿ 1999 American Geophysical Union