Specimens of Mysis relicta from Pojoviken Bay (Finland) were divided into two groups, one kept at 4°C, the other at 14°C, in total darkness. Immediately following a 1 h exposure to noon sunlight both 4°C and 14°C animals displayed strongly reduced visual sensitivities. In both groups pre-exposure levels were regained in about two days, but apparently along slightly different routes. Slopes of V/log I curves hardly changed throughout the time of observation in the 14°C material, suggesting an adaptation to brighter light levels without undue stress responses. In the 4°C animals, however, there appears to have been not only a longer initial delay before recovery commenced, but slopes of V/log I curves indicated that these animals had reacted with depression to the bright light and needed time to regain their pre-exposure value. The results suggest that recovery is a two-stage process in which biochemical reactions and structural phenomena interact. The results, when compared with similar observations on Lake Pääjärvi specimens, underline the view that different localities may have populations of Mysis relicta which differ from each other in photophysiological characteristics.