ABSTRACT The vitality of 64 lichen species (107 individual lichen thalli) growing under different light climates in an Araucaria forest in South Brazil was analyzed by chlorophyll fluorescence. Study sites were grouped according to their local light availability under full sunlight (about 2200 µmol m-2 s-1): 1 = low light, up to 20 µmol m-2 s-1; 2 = medium light, 20 to 100 µmol m-2 s-1; and 3 = high light, more than 100 µmol m-2 s-1. Maximum quantum yield of photosystem II, as shown by Fv/Fm of dark-adapted samples, was mainly between 0.3 and 0.7, with extremes of below 0.1 and up to 0.85. On average, yields were highest with low light availability (0.66). Groups 1 and 2 were not significantly different from each other, but groups 1 and 3, as well as groups 2 and 3 were. After dark adaptation, lichens were exposed to different light intensities by means of a chlorophyll fluorometer. The results show that low light-adapted lichens exhibit the highest sensitivity to excess light, as was also indicated by the data for non-photochemical quenching. Thus, shade-adapted lichens are obviously well protected from possible damage caused by excess light, which is important when exposed to sun flecks.