This study investigates the phytotoxicity of ozone during periods of natural frosts following long-term fumigation. Heather (Calluna vulgaris (L.) Hull.) plants were fumigated with ozone for 6 months at 70 nl I−1, for 8 h d−1, 5 d a week in open-top chambers. Natural frosting occurred within the chambers towards the end of this period (5–27 December 1995). Plant responses to the ozone/natural frosting episode were recorded in January 1996. We show conclusively that naturally occurring frost exposure results in increased damage and mortality for ozone-treated plants, which could have important implications for the function of heathland ecosystems.