Sporadic Na (NaS) layers have been observed frequently at low- and high-altitude sites primarily in the Northern Hemisphere. NaS layers are very dense and very thin (~1 km FWHM) and can form over periods ranging from a few minutes to several hours. Radar and lidar observations by our groups at Arecibo have shown conclusively that NaS layers are linked to the formation of sporadic E (ES) layers. Large NaS events were observed on several flights during ALOHA-90 to extend over horizontal distances of approximately 1800 km. Atmospheric tidees and gravity waves were observed to have an effect on the vertical motion of these layers. We describe the 22 March event which was observed from Maui (21¿N) to near the equator (5¿N) and the 27 March event from Maui (21¿N) to near the equator (5¿N) and the 27 March event which was observed from Maui (156¿W) to almost the International Data Line (~172¿W). ¿American Geophysical Union 1991