Zwackhia viridis is a crustose lichen being found in forests throughout Poland. It belongs to rare species being labelled primeval forest lichens which are associated with large forest complexes. The eastern populations of Z. viridis were examined using RAPD technique in order to check whether geographic distance affects genetic diversity of the populations under study. The analysis of seven populations of that species showed large intraspecific diversity. The greatest genetic similarity occurred between populations from the north-eastern area of the country, about 300 km away from each other. In dendrograms, only these populations have simple matching distance greater than 50%.