The present study attempts to deduce the levels of air-pollution from its effects on epiphytic lichens in the Miinsterland caused by the emission source of the Ruhrgebiet adjacent in the South to the investigated area. For the collection and determination of the values the author has used the lAP-method developed by De Sloover & Le Blanc (I968). A distribution map has been constructed by examin ing the lichen flora of 117 sites located all over an area encompassing about 2 3000 km . Since Malus domestica is relatively abundant in the territory and as it frequent ly stands isolated in the...