A large sporadic Na layer was observed near 109 km during active aurora at Poker Flat, Alaska, on the night of 12--13 January 1996. This sporadic layer is significantly denser and broader than previously reported thermospheric sporadic layers; having a peak concentration equal to the peak concentration of the background Na layer, a FWHM of 4.7 km, and a column abundance of 40% of the background Na layer. The appearance of the NaS layer coincided with a significant OH temperature gradient across the sky but did not coincide with the presence of a discrete arc overhead nor a distinct signature in the local magnetometer data. The presence of such a large amount of neutral Na at these altitudes contradicts the current models of NaS layer formation. The observations are discussed in the light of other observations, current models, layered auroral phenomena, and meteoric ablation. ¿ American Geophysical Union 1996