The nature of the delayed chlorophyll fluorescence (DF) in algae, which occurs already after the cessation of illumination due to the energy released during the reverse reactions of the primary photosynthesis photoproducts in the reaction center of the photosystem 2, is considered. The use of this fluorescence in algae for biotesting of contaminants is shown to be promising. The advantages of using DF for express assessment of water toxicity, especially drinking water, are short durations of tests and, accordingly, the minimized negative impact of physicochemical changes in the medium on the results, as well as the need of small amounts of test objects. The other advantage of DF is that only living cells with active photosynthetic reaction sites are measured, which increases the sensitivity of toxicity tests. This method provides additional information about the most important process in the cell - the energization of photosynthetic membranes and the formation of ATP associated with this.