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). As a first stage in the interpretation of the results, identically treated parallel samples were used, under controlled laboratory conditions, to determine the dependence of net photosynthesis (NP) and dark respiration (DR) on light, CO2 concentration, thallus hydration and temperature. Results of these measurements are communicated in the present, first part of a series of publications. Key words: Lichen, photosynthesis, water content, light, temperature, Lecanora muralis.