Calluna vulgaris (L) Hull. were fumigated in open-top chambers with ambient charcoal filtered air, or 70nl 1•1 (70 ppb)ozone, for 8h per day. 5 days a week. Ozone treatment during the winter increased the frost sensitivity of plants as measured by electrolyte leakage following a controlled frosting regime. Root grow1h was decreased and the allometric root/shoot ratio(k) was reduced by the ozone treatment. There was no significant effect or ozone upon net photosynthesis. There was a significant increase in anthocyanin content in ozone fumigated plants, but no effect on chlorophyll content