Abstract: This invention provides a process and an apparatus for the detection of toxicity in surface waters, by which the fluorescence of a water sample is measured in that the correlation, especially the ratio, between the prompt fluorescence and retarded fluorescence of the water sample is determined, which contains a toxicity-sensitive bioorganism. The apparatus comprises a combination of a first fluorescence measuring means for measuring prompt fluorescence and a second fluorescence measuring means for measuring retarded fluorescence as well as a signal correlation means for correlating the output signals of two fluorescence measuring means with each other to form an output quantity indicating the toxicity of the water sample.