The lichens play an important role in decomposition of the rocky substrate, through the chemical weathering of their substances. In this paper, we wanted to determine the influence of gamma rays on usnic acid production by Cladonia substellata. Samples of lichens were exposed to gamma rays from a Co-60 source, receiving different doses on gamma irradiator. After irradiation, the lichens were packed over shredded limestone. For 90 days the lichen samples were collected and analysed by high performance liquid chromatography. The rocks samples were analysed by X-ray diffractometry. We realised that C. substellata increases the usnic acid biosynthesis when the gamma radiation dose is increased, but there is a limit for it. The degrading effect of the usnic acid on limestone was proportional to the produced amount of the substance, which could be extrapolated for natural conditions, where excessive radiation may have an influence on pedogenesis and ecological succession.