VHF radar observations of polar mesosphere summer echoes (PMSE) have been carried out at the ALOMAR facility near Andenes (69.3°N, 16.0°E) using the ALOMAR SOUSY radar during the four summer seasons from 1994 until 1997. First PMSE occur near the middle of May, and last PMSE are normally observed near the end of August. The mean seasonal variation of the occurrence rate of PMSE is characterized by a steep increase in late May/early June, a maximum some days after the summer solstice and a slow decrease starting during the last decade of July. Diurnal variations of PMSE strength as well as of the height of the maximum backscattered power are dominated by a pronounced semidiurnal variation. This behavior is markedly influenced by temperature changes induced by tidal waves in the mesospheric wind field.