Despite the geographical isolation of Antarctica, more than half of the lichen flora of this region is composed of cosmopolitan and bipolar species, many of them restricted to the maritime Antarctic. Since low temperatures are considered a major limiting factor for plant growth, we studied the ecophysiological behaviour of some cosmopolitan macrolichens under natural summer conditions in the maritime Antarctic and we compared the results with those obtained in the laboratory under controlled conditions. Three species with different growth forms were selected: P saxatilis (L.) Ach., foliose, Pseudephebe pubescens (L.) M. Choisy, fruticose and Umbilicaria ana (Zahlbr) H. Magn, umbilicate. Gas exchange of these species was measured throughout 14 days of metabolic activity under natural conditions of light, hunudit>', hydration and temperature on Livingston Island (South Shetland Islands) in January, February and December 1995. The carbon balance during the period investigated was negative for all three species. Carbon balance was highly affected by high dark respiration rates. However, although growing in the same locality , the temperature dependency' of net photosynthesis differed substantially in U. nylanderiana and P. saxatilis with a more pronounced adaptation to cold temperatures in U.nylanderiana. Chlorophyll content was extremely low in all of the investigated species