Carbon dioxide exchange of the epilithic, green algal lichen Lecanora muralis and the weather conditions in its natural habitat in the Botanical Garden, Würzburg (Bavaria, Germany), were monitored by means of an automatic cuvette. The continuity of the data over a total of 15 months of measurements enabled integrals to be calculated of diel, monthly, seasonal, and annual sums of photosynthetic carbon income (σNP), respiratory carbon loss (σDR) and carbon budget (net primary production, σC = σNP - σDR). Key words: lichen, photosynthesis, respiration, carbon budget, seasonality, Lecanora muralis.