Abstract: The invention relates to the field of optical biosensors and facilitates detection of mobile microorganisms in a liquid with a high degree of sensitivity even in the presence of a large number of brownian particles. The method in essence involves measuring, within a frequency range restricted at the lower end, the mean amplitude of the fluctuation in intensity of laser radiation scattered by the particles in the liquid, this being proportional to both the concentration and mobility of the particles. The turbidity of the liquid, which is proportional only to the particle concentration, is also measured and the presence of mobile microorganisms is assessed by comparing the two signals.