The first daytime lidar observations of the mesospheric Na layer at Urbana, Illinois (40¿N, 88¿W), were made in January and March 1986. The data show strong 12-hour oscillations of the Na density and layer centroid height which appear to be caused by the semidiurnal tide. The peak-to-peak variations in column abundance and centroid height were of the order of 100% and 3 km, respectively. The vertical wind velocities were inferred from the temporal variations of the density gradients on the layer topside. The inferred wind amplitudes and vertical wavelengths for the semidiurnal tide were ~30 cm s-1 and ~50 km, respectively. The maximum upward vertical velocity occurs at about 1100 LST and 2300 LST. ¿ American Geophysical Union