Twenty-four-hour observations on the behavior of white water-lily (Nymphaea candida Presl.) flowers were conducted. The observations were made during different photoperiods in the Middle Russia (natural photoperiod, light flash in the dark daytime, absence of light daytime) and in the Northern Karelia (polar day). We have elucidated that: (1) degree of flower openness changes rhythmically both in Middle Russia and in the Northern Karelia, while the degree of flower’s submergence remains almost constant, (2) flower behavior, typical for Middle Russia, can be also observed during the polar day, (3) deviations from natural rhythms in the absence of dark period are less prominent then in the absence of light period. Movements of water-lily flowers appear to serve for pollination optimization.