Extensive biological analysis of raw and biologically treated wastewater demonstrated the presence of yeasts, acid fast, staphylococci and total coliform grganisms, in all tested samples, reached average values of 5.6x10°, 1.3x10', 1.9x10' and 1.4x10' respectively in raw, wastewater. These figures are reduced to 5.6x10°, 2.1x10, 4.3x109 and 3.6x10' during biological treatment and primary sedimantation. The relative resistance of these organisms to chlorination was found to be: acid-fast >yeast > staphylococci > coliforms. Acid-fast, yeast or staphy lococci organisms can represent a valid indicator of chlorination efficiency rather than coliforms.