Solar radiation has a pronounced effect on photoorientation, motility and pigmentation in the freshwater dinoflagellate, Peridinium gatunense. Photoorientation (positive phototaxis) is impaired even after short exposure times and is totally inhibited after about 2.5 h. The percentage of motile cells decreases after short exposure times and after 4 h most cells are immotile. Likewise, the mean velocity decreases after an initial light-induced increase (photokinesis) under solar radiation. Solar UV-B (280-320 nm) radiation seems to be the major cause for the effects, since cutting off shorter wavelength by an ozone cuvette or WG filters prolongs the tolerated exposure times. Possible UV-B targets are the photoreceptor pigments since solar radiation causes a massive bleaching of the cells within a few hours.