Observations with a 46.5-MHz radar system at Aberystwyth, Wales (52 ¿N, 4 ¿W) over the period February 1990 to May 1994 have shown a pronounced seasonal variation in the height range of mesospheric echoes. These were observed over the range 64--84 km in all seasons, but those at still greater heights were confined to summer months. Coordinated radar and lidar observations at the same site during 1993 have shown that these intense echoes in summer, analogous to polar mesosphere summer echoes (PMSE), are associated with low mesopause temperatures; the weaker echoes observed in winter seem to be associated with regions of superadiabatic lapse rates; and echoes are observed both above and below the maxima in temperature inversions commonly observed below the mesopause. The types of instability implied in these associations of echo height and temperature structure are considered. ¿ American Geophysical Union 1996