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In order to ascertain whether the major species of continental antarctic macrolichens are photosynthetically active during summer conditions, the chlorophyll fluorescence of three lichen species [Umbilicaria decussata (Vill.) Zahlbr., Pseudephebe minuscula (Nyl. ex Arnold) Brodo and Hawksw. and Usnea sphacelata R. Br.] was monitored in the vicinity of Casey Station, Wilkes Land, continental Antarctica using a PAM-2000 modulated fluorescence system. Lichens were studied when in equilibrium with the atmosphere as well as when moistened by snow showers. Photochemical quantum yield was estimated as DELTAF/F(m)’ and related to thallus water content as well as microclimatic conditions. Lichens were photosynthetically active only when moistened by snow fall or by run-off from snow melt. The levels of photosynthetic activity in the field for all species were influenced by microenvironmental conditions and patterns in response were site and species specific. Highest levels of photosynthetic efficiency occurred when thalli were at intermediate water contents. Photosynthetic activity was reduced by cold as well as warm, bright conditions. Highest thallus water contents occurred during the middle of the day after substantial falls of snow. P. minuscula maintained highest thallus water contents at all sites and appears to have a high water compensation point which is related to its observed distribution patterns. Umbilicaria decussata studied in the laboratory did not become photosynthetically active even when exposed to 95% relative humidity (RH) for 51 h and, when dehydrating after artificial wetting, showed an optimum thallus water content for photosynthesis of ca 90% dry weight and a thallus water compensation point of about 35% dry weight. In the field U. decussata did not become photosynthetically active except when moistened by snow. Usnea sphacelata exposed to the atmosphere had low thallus water contents (ca 30%) which was not related to RH. The results indicate that the lichens are photosynthetically inactive for most of the summer period and are totally reliant on snow as a water supply. This is important when modelling carbon gain and growth rates of continental antarctic lichens.