The purpose of the research was to explore the feasibility of using Bel W3 tobacco plants for biomonitoring O3 pollution in the Greater Athens Region and other parts of the country and also to obtain some information on O3 sensitivity of Greek tobacco varieties. Bel W3 plants were exposed to ambient O3 pollution in several sites and at distances of up to 40 km from the city. Once a week, throughout the lives of the plants, data on plant height, number of leaves, length (L) and maximum width (W) of each (numbered) leaf were obtained. Also the surface area (E) of each leaf was estimated using the derived relationship : E = 0.103 + 0.63 (L x W)Expressing E as % of the leaf's final area, the Leaf Stage of Growth (LSG) was obtained. Ozone effect on each leaf was recorded, on a 0 to 5 scale, as an Index of Leaf damage (ILD). The data show that all plants in all sites were damaged by ambient O3, but not all leaves were equally sensitive. Practically no leaves were damaged before reaching about 85% of their final size. The relationship between ILD and LSG was : ILD = 29.42-10 x e0.3456(LSG)Yet, old overmature leaves and leaves under water stress were resistant. Leaves with partially broken central nerve did not show O3 damage beyond the point of breakage. Several Greek tobacco varieties were fumigated with O3. Preliminary results indicate that the varieties KK 6/5 (1988), M.V. (1987) and B(ksi) 2/a (1986) were sensitive, while the varieties SIGMA 53 (1989) and KPI 14/a (1989) were resistant.