During the early morning of October 31, 1988 two large sporadic Na(NaS) layers were observed near the mesopause above Urbana, IL (40¿N, 88¿W) with a Na lidar system. The layers began forming near 102 km at 0026 LST and 0110 LST and moved downward with vertical velocities as high as 4 ms-1 before dissipating between 94 and 96 km. The duration of each layer was approximately 80 min. The layers were narrow (~1 km FWHM) and dense with maximum densities approaching 7800 cm-3. The characteristics of these two NaS layers are very similar to those of similar phenomena observed recently at Andoya, Norway [von Zahn et al., 1987> and Mauna Kea, Hawaii [Kwon et al., 1988>. Lidar observations of the mesospheric Na layer have been conducted routinely by several groups at mid-latitudes for almost 20 years. Although large NS layers now appear to be relatively common at low- and high-latitudes, to our knowledge the two layers described in this letter are only the second observation of this puzzling phenomenon at mid-latitudes. ¿ American Geophysical Union 1989