The wastewater treatment by infiltration basins used in Créances (France) allows for the protection of the quality of surface waters. The infiltration leads to a total removal of suspended solid and of 80 to 85 % of COD initially present in the effluent. In summer N and P removal corresponds to two-thirds of the concentration of NH4+ and PO43− but in winter no removal is observed for these elements. Whatever the season removal of total coliforms, faecal coliforms and faecal streptococci is high and reaches 6 logs. A part of the contamination is transferred by renovated water into the unconfined ground water which lies at a depth of 6 m below the bottom of the basins. The effect of infiltration on the ground water quality includes an increase in overall mineralisation of the water and especially as regards nitrogen, anionic detergent and boron levels. Otherwise, difficulties of managing the plant result from the fast clogging of the basins because of a low removal of suspended solids in the pretreatment stage and from the climatic conditions which increase the time to dry out the cake.