Bacterial contamination of raw and treated wastewaters has been studied for 15 purification plants. Results have permitted the evaluation of bacterial effectiveness of different treatment modes. Best elimination rates of indicators (faecal contamination test-germs) were obtained by extensive treatments. Aerated lagoons were not usually more efficient than natural ones. Oxidation ditches were classified after stabilization ponds with a bacterial reduction of 2 log units. Biofiltration seemed to have the same effectiveness than activated sludge which were slightly more efficient at low-charge. Global elimination of Faecal contamination test germs by conventional intensive treatments is 1-2 log units.