Cryptosporidium, protozoa naturally found in the superficial waters is responsible for severe digestive disorders among human and animal. Retaining efficiently these parasites in the water supply treatments is a major concern for public safety. This study is divided in two parts: • Evaluating the contamination level of superficial waters in the west of France. • Assessing the efficiency of the parasites retention during the clarification process to evaluate the influence of the clarification and filtration conditions both • - In real context on 3 plants. • - On a pilot. The results on the field show the influence of the rainfall on the contamination levels. The results on the 3 plants show ratio of parasites retention in the filtered waters higher then 99%. The best results on the pilot (concentration in oocystes of Cryptosporidium decreasing by 2.6 logarithmic units) where obtained by using ferric chloride as coagulant and sand of 1.2 mm diameter for filtration. The retention is slightly better a cationic polyamine as coagulant additives. The washing and the maturation of filters are critical steps in the Cryptosporidium retention in the water treatments.