The presence of tropospheric ozone was monitored by estimating necrotic flecks on the leaves of the ozone-sensitive tobacco variety Bel W3 in Estonia in 1990 and 1991. The indicator and control plants (variety Samsun) were planted in four localities (K~smu, northern Estonia; Tartu, central Estonia, with two sites and Kooraste, southern Estonia). The percentage of the leaf area covered with necrotic flecks (necrotic index of the leaf) was determined individually for each leaf from mid-June until the end of September. The mean daily increment of the necrotic index for the period between two observations (typically 6-14 days) was calculated for each plant (NII~ 0 and for the site (NII~it~). The latter parameter indicates the variability of ozone episodes during the vegatation period. Ozone damage to leaves displayed an irregular pattern throughout the vegetation period, the values of NIIs~to ranging from 0 to 0.3 (Kooraste, 1990) and 0.9 to 2.8 (K~mu, 1991). The average NII,i ~ for the vegetation period was highest in northern Estonia (0.77 in 1990 and 1.47 in 1991) and lowest in southern Estonia (0.07 in 1990). High ozone damage in K~smu can be explained by the close location (70-80 km) of the experimental site to major thermal power plants of Estonia and cities (Tallinn and Helsinki). Higher ozone damage in all the observed sites in 1991 as compared to 1990 can be accounted for different conditions for ozone formation.The number of sunny hours was 27% higher in 1991. In July the difference in sunny hours was almost twofold. It is concluded that ozone damage to plants is likely to occur in sunny summers in Estonia.