Introduction The lichens diversity of the Republic of Moldova has been studied over the past 75 years. Currently, the lichen diversity comprises 197 species and varieties, of which 23 species are reported nationally for the fi rst time and 40 species are recommended as reliable indicators. We presents a Lichens Toxi-tolerance Scale towards SO2 concentration in the air and Air Quality Assessment Scale, taking into account the specifi c diversity, abundance and toxi-tolerance. Material and methods Of importance for theoretical science is as well the placement of lichen species into a systematic classifi cation [3]. The classifi cation system used follows several works [1] and scientifi c names are according to Kondratyuk et al. [4]. Currently there is much information on disturbances to the vital activity of lichens caused by atmospheric pollutants. This has allowed some authors to specify gradations of SO2 concentration [5]; [6]; [2]. Results and discussions In 2001, the laboratory of Environmental Impact and Ecobioindication was founded at the National Institute of Ecology, with research activities focused on lichen species as bio-indicators. A particular emphasis was put on bio-indicator features of lichens in forest and petricolous ecosystems. The Republic of Moldova Lichens Register was completed with 23 new species. By applying the a.m. method we established the presence of 40 lichen species as indicators of air quality: 3 species - with I degree, 16 species - II, 16 species - III, 3 species - IV, 2 species - degree V of toxi-tolerance, which may certainly form the basis of biological monitoring in our country. The frequency of these species in forest ecosystems is rather high, which guarantees the use of the same species for the forest sector, but also gives the opportunity to connect to the National and European Ecological Network, as many species are common to the European environment. Proceeding from the abundance and toxi-tolerance of the indicators we have elaborated a scale with gradations for the evaluation of air quality. Conclusions The Republic of Moldova Lichens Register includes c. 200 species and varieties (23 noted by the author). We propose 40 indicator species sensitive to air pollution by SO2, and two criteria for the evaluation of the state of environmental components.