Lidar observations at Aberystwyth (52.4¿N, 4.1¿W) have shown layers of enhanced backscatter at heights just below the mesopause during four nights in late June and early July, 1993. The characteristics of these layers and their association with noctilucent clouds indicate that they are comprised of ice crystals. An examination is made of the temperature prior to the formation of a layer, the rates of descent of layers, and their integrated backscatter coefficients. The results are discussed in terms of the size of the crystals giving rise to backscatter, the ambient water mixing ratio, and the equivalent water vapour represented by these crystals.