Abstract: The present invention provides a process for the control of a biological rification stage of the aerobic activated sludge type, wherein at least one of the micro-organisms most frequently present in the activated sludge is continuously monitored with regard to the amount thereof in that, in a representative sample from the activated sludge and/or from the inlet of the activated sludge tank, this micro-organism is bound to fluorescence-labelled antibodies directed against the chosen micro-organism or this micro-organism is allowed to react with a fluorogenic substrate by means of a special metabolic ability, the amount of the thus fluorescence-labelled micro-organism is determined by flow cytometry and, at the same time, the total amount of the micro-organism present is determined by scattered light measurement and/or coloration of the DNA and, depending upon the measurement values thus obtained, the amount of at least one particular micro-organism and/or the growth conditions for this micro-organism is regula