Dried thalli of the lichen Cladonia foliacea were progressively extracted in different solvents (light petrol, diethyl ether, chloroform, methanol, water at pH 7 and at pH 8) and tested for their allelopathic activity in vitro on spore germination and sporcling growth of the mosses Funaria hygrometrica, Tortella flavovirens, Bryum dunense and Pleurochaete squarrosa. They caused a decrease in the percentage of spore germination and potential growth. Two inhibiting substances were present in the extract: the usnic acid and an aliphatic aldehyde linked to a chromophore. Moreover, (alditols as arabitol, mannitol, etc.) molecules stimulating growth of the mosses were also isolated.