Repair of DNA damage induced by ultraviolet-B radiation (UVBR) was investigated in the sublittoral red alga Rhodymenia pseudopalmata at different temperatures, using immunofluorescent detection of thymine dimers. Photoreactivation of thymine dimers was completed within about 3 h at 6, 12 and 18 °C in the presence of ultraviolet-A radiation (UVAR) and photosynthetically active radiation (PAR) but no repair was found in dark-incubated fragments after 16–17 h. In plants previously exposed to a low UVBR dose (biologically effective dose: BEDDNA300 = 1·6 kJ m−2; unweighted irradiance = 0·72 W m−2 for 4 h), photoreactivation started within the first hour under UVAR+PAR at 18 °C but repair only started after 1–2 h at 12 and 6 °C. At 6 °C, repair was more efficient after exposure to a high UVBR dose (BEDDNA300 = 3·9 kJ m−2; unweighted irradiance = 0·78 W m−2 for 5 h) than to a lower dose (BEDDNA300 = 1·6 kJ m−2); no such difference was found at 18 °C. It is concluded that R. pseudopalmata is able to repair DNA damage induced by high UVBR doses efficiently.