The influence of solar radiation (PAR and UV) on fresh water mesocosms located at three different altitudes (Erlangen, 279 m a.s.l.,Wank, 1780 m a.s.l., Zugspitze 2664 m a.s.l.) was investigated from May to September in three successive years from 1995 to 1997.  The mesocosms of 225 1 contents were filled with a defined mixture of water from the Main-Danube-Canal and sediment from the Seebach near Erlangen, Germany.  Mineralization of the mesocosms and replacement of evaporated water guaranteed a constant nutrition level.  One set of mesocosms was exposed to solar radiation (295 nm cut-off filter, control) another to photosynthetic active radiation (PAR) plus UV-A and a third to PAR only. Samples were monitored every three weeks.  The specific effects of UV- B radiation was obvious at all measuring sites and in all years.  The mesocosms exposed to PAR+UV-A+UV-B showed a lower organism concentration than the PAR plus UV-A and PAR only mesocosms.  The species composition of the organisms or the distribution of the organisms in the water column was not significantly influenced by the different radiation treatments, but the sequence of species was different in dependence of the radiation regime.  The chlorophyll a contents, total organic carbon (TOC) and particulate organic carbon (POC) increased during each measuring period in parallel to the water temperature.  The light penetrating into the water column was rapidly attenuated, but no pronounced vertical distribution of phytoplankton could be observed.  The ratio of UV radiation to PAR was higher on the Zugspitze and Wank than in Erlangen, but an effect on the mesocosms could not be separated from the temperature effect.