Aims: Evaluation of the efficiency of peracetic acid in the disinfection of wastewater in a large treatment plant. Methods and Results: Over a period of 18 months 30 sample collections were made, each consisting of three samples taken from: raw incoming sewage, secondary effluent (after 10-12 h) and secondary effluent disinfected with 1.5-2 mg l(-1) of peracetic acid (contact time: 20 min). Total coliforms and Escherichia coli declined from 10(7) MPN 100 ml(-1) in the raw sewage to 10(2) in the disinfected effluent and the enterococci fell from 10(6) MPN 100 ml(-1) to 702 MPN 100 ml(-1). The reduction of bacteria increased with the rise in temperature and decreased with the rise in BOD5. Conclusions: Disinfection with peracetic acid reduced levels of faecal contamination by 97%, thus attaining the limit recommended by current Italian law (Escherichia coli less than or equal to 5000 MPN 100 ml(-1)) for discharge into surface waters. Significance and Impact of the Study: The process of disinfection with peracetic acid is easier to manage than other more common methods and the tests performed confirm that from the bacteriological point of view good results can be obtained for urban effluents.