The photosynthetic perfonuance of four lichen species growing as endoliths or as cryptoendoliths in sandstone was investigated. Measured were net photosynthesis and dark respiration of Lecidea aff. sarcogynoides and Sarcogyne cf. austroafricana in Tshipise sandstone from Mutamba Ranch (northern Transvaal) in a semi-arid region of South-Africa and Lecidea aff. sarcogynoides, Lecidea confluentula and Lithoglypha aggregata in Clarens sandstone from the Golden Gate Highlands National Park (eastern Orange Free State) which is located in a warm-temperate mountain region. CO2 exchange within +5°C and +30°C (5K temperature steps) and a range of irradiances between 0 and 700 pmol m'2 s'1 PPFD at the rock surface was measured in the laboratory under artificial light An adaptation to the general temperature level was evident with Lecidea aff. sarcogynoides. The photosynthetic response of the other species was not clearly proportional to the general temperature conditions of the habitat. Light gain at the level of the photobiont is drastically reduced depending on the depth of the algal layer in the rock and on the density of the substratum. Because of the light reduction only the samples from Mutamba reached light saturation of net photosynthesis at moderate irradiance level (less than 600 pmol m'2 s'1 PPFD) at the rock surface and low temperature (+5°C). Light reduction in the rock was also apparent from the comparatively high light compensation level which varied between 15 and 300 pmol m “ s'1 PPFD at the rock surface, depending on temperature. The lichens reached net photosynthetic rates of maximally 1.14 to 2.47 pmol C02 in2s'1 in the laboratory and 0 08 - 0.97 pmol CO: nTs'1 in the field Thus, Lecidea confluentula. Lecidea aff. sarcogynoides and Lithoglypha aggregata revealed a productivity which is in the order of magnitude of crustose and foliose epilithic desert species and Sarrcogyne cf. austroafricana within that of Antarctic cryptoendoliths. Measurements were also taken in situ on rocks within the area of Mutamba farm in the tropical semi-arid northern part of Transvaal province. South Africa. Although being in the rainy summer season, there was no rain event during the period of our stay. Moisture in the rocks was generally too low for the activation of the lichens. Spraying with water was effective only in the cooler morning hours and photosynthetic activity lasted for less than 30 minutes under the prevailing environmental conditions. It is concluded that these lichens are only active if rain or dew water keeps the rock wet which happens under cloudy or overcast skies or in deep shade.