The inactivation of a large-focus-forming variant of hepatitis A virus (HM-175) by ozone was investigated. Experiments using mainly single-particle virus preparations suspended in phosphate–carbonate buffer were conducted over a range of pH levels (6–8) at 4 °C. Viral enumerations involved the use of a radioimmunofocus assay. While some tolerance to lower (i.e., 0.1–0.5 mg/L) ozone residuals was noted, the exposure of virus particles to ozone concentrations of 1 mg/L or greater at all pH levels resulted in their complete (5 log) inactivation within 60 s. The pH-related effects that were observed were not considered to be significant. Key words: hepatitis A virus, ozone, disinfection.