A comparison was made between the effi ciency of low doses of peracetic acid (PAA: 1.5 mg/l) and chlorine dioxide (ClO2: 1.5 and 2.0 mg/l) in the disinfection of secondary effl uents of a wastewater treatment plant. Peracetic acid was seen to be more active than chlorine dioxide and less infl uenced by the organic content of the waste. Both PAA and ClO2 (2.0 mg/l) lead to a higher reduction in total and faecal coliforms and E. coli than in phages (somatic coliphages and F-specifi c RNA bacteriophages) and enterococci. Detection of faecal coliforms and E. coli should therefore be accompanied by a search for these more resistant microorganisms when assessing the conformity of wastewater for irrigation use, or for discharge into surface waters. Coliphages are also considered suitable indicators of the presence of enteric viruses. Although the application of low doses of both disinfectants offers advantages in terms of costs and produces not signifi cant quantities of byproducts, it is not suffi cient to obtain wastewater suitable for irrigation according to the Italian norms (E. coli < 10/100 ml in 80% of samples and <100/100 ml in the remaining samples). Around 65% of the samples, however, presented concentrations of E. coli lower than the limit of 5,000/100 ml established by Italian norms for discharge into surface waters.