Schistidium antarctici is the commonest of five bryophytes known in the Windmill Islands area of Wilkes Land, Greater Antarctica. In moist habitats it forms closed carpets, but in dry sites it develops a short cushion growth form. Carbon dioxide exchange of both a mesic (Sm) and a xeric growth form (Sx) was investigated by means of an IRGA system in the field near Casey Station under natural light and simulated ambient or controlled temperature conditions in the plant chamber. The chlorophyll content in Sm was three times higher than in Sx. The light compensation point of Sm was lower than in Sx. The data for photosynthesis and dark respiration were computed by means of non-linear and linear regression analysis. Sm was more productive and had a wider temperature range of positive net photosynthesis than Sx under similar conditions. Dark respiration per gram of the whole moss sample was identical in both ecodemes. A decline of the photosynthesis curves at quantum flux densities above 500 μmol m-2 s-1 PAR indicated a photoinhibitory effect in Sm. Sx was even more sensitive to high irradiance levels. Photoinhibition was not apparent in laboratory measurements under artificial light. According to our field measurements the photoinhibitory effect increases with increasing temperature. Moisture loss was avoided during the experiments by water supply from the bottom and frequently spraying the moss samples with water. In the natural habitat the desiccating effect of solar radiation is important, as it quickly causes photosynthesis to cease. The moss will dry out sooner in a xeric habitat than in one which is continuously moist. Consequently, the mesic Schistidium might particularly be subjected to photoinhibition by bright sunshine.