Wet thalli of Lobaria pulmonaria collected in La Palma (Canary Islands) were exposed to high temperatures (22°- 44°C). They were heated in the dark either in a wet atmosphere (20 min) or in a water bath (submerged for 5 min). After cooling photosynthetic parameters were determined. The chlorophyll fluorescence parameters Fv/Fm (potential electron transfer in photosystem II), AF/Fm’ (- apparent electron transport rate). qP (photochemical quench) and photosynthetic CL-evolution rate strongly declined at heat treatment beyond 36°C for both treatment types. Direct effects on pigments (chlorophyll degradation, zeaxanthin formation) were not found. It is concluded, that photosy stem II and possibly photosynthetic dark reactions of the photobionts are severely damaged at temperatures below 40°C. Obviously, the heat resistance of the Canary Island material of Lobaria pulmonaria in the wet state is as low as reported earlier for central and north European specimens (Lange, 1953) and similarly low as for the desert lichen Ramalina maciformis (Lange, 1965). Considering their natural habitat, this is only possible as the lichens get dry when the temperature rises