The effects of variation in temperature, photon flux density and thallus water content levels on CO2 gas exchange were examined in two tropical lichen species: Coenogonium in (et-plexum, an algal dominated ascolichen, and an alpine form of Dictyonema glabratum, a basidiolichen with a cyanobacterial symbiont. The basic C02 gas exchange patterns clearly reflect the habitats from which the material was collected. For example, within the conditions of our experiments, D. glabratum exhibited a temperature optimum at 10°C, whereas C. interplexum had an optimum of 20°C. Obvious net photosynthetic (NP) depression did not occur in D. glabratum at values from 150 to ~350% water content, nor did it occur in C. interplexum at highly saturated conditions of up to 875% w ater content, but did occur in that lichen at supersaturated concentrations of 1600%. Dark respiration (DR) in D. glabratum showed a continuous strong increase with increasing water content, whereas in interplexum, DR values were fairly stable from 500-2000%. and possibly at lower water contents. A comparison of DR and NP temperature dependence in D. glabratum suggests significantly higher rates of gross photosynthesis (GP) with increasing temperature from 15°C to 25°C, the maximum temperature examined for that species. The pattern of gas exchange in D. glabratum is quite different from other investigations of specimens of this species from substantially lower elevations. However, the magnitude of the response is much higher than that of at least one other study. In general, NP rates and thallus hydration levels and responses in D. glabratum are much lower than those found in studies of lower elevational forms. NP values in C.interplexum are much higher than what has been found previously in D.glabratum. The lack of photosynthetic depression at high water contents in C. interplexum is similar to that of D. glabratum from previous studies