CO2 exchange of the epilithic, green algal lichen Lecanora muralis was monitored continuously for 15 months by means of an automatic cuvette in its natural habitat in the Botanical Garden, Würzburg (Bavaria, Germany). Simultaneously weather data and chlorophyll fluorescence were recorded. The periods of metabolic activity for the lichen were restricted because of the low water holding capacity of its thin thallus and high desiccation rates. The total activity was only 35.6% of the year made up of net photosynthetic CO2 uptake, 16.7%, and respiratory CO2 release, 18.9%. This was substantially less than found for gelatinous or foliose lichens under similar conditions. Key words: lichen, photosynthesis, diel course, water content, Lecanora muralis.