Wastewater containing indigenous bacteria and enteric viruses was ozonated in a contact column with counter-current circulation of ozonated air and water. The mean slope of the inactivation curve for indigenous viruses is about half that observed forE. coliand fecal streptococci. For 3 rag osub3;/l, in the presence of residual ozone and a contact time of 19 min, inactivation of 1 log for the viral population and 2 logs for the fecal bacteria are observed. A negative correlation exists between inactivation and organic content.