It is now known that since cyanobacteria (blue-green algae) occur in both swimming and drinking water supplies, and lakes and rivers, they represent an increasing hazard to animal life and human population. Moreover, high algal contents pose also a number of operation problems for water purification plants. The objective of the work is to study the elimination of a Microcystis strain of cyanobacateria by the use of an ozoflotation process which associates the oxidizing properties of ozone and the physical aspects of flotation. The functioning and the efficiency of a pilot unit is presented according to such parameters as: ozone dose, flow rate, coagulants and raw water quality. The use of ozone in pretreatment leads to an inactivation of the algal cells. Experiments let us calculate the specific ozone utilisation rate of Microcystis and the [C.t] (ozone concentration, contact time) curve is determined versus algal removal. Under real conditions, a previous coagulation stage is necessary; best results are obtained with ferric chloride. Preozonation is also of influence on the enhancement of the coagulation efficiency. Association of the ozoflotation process and bilayer filtration can solve the algae problems of waters presenting low turbidity and low organic content, and improve water quality.