CO2 gas exchange and microclimatic conditions of Leptogium azureum (Sw. ex Ach.) Mont., a foliose lichen, were investigated in the lowland tropical forest of Barro Colorado Island, Panama. Thalli growing epiphytically in the forest canopy or epilithically on boulders in the understory were studied. Under the prevailing temperature and humidity conditions, thallus water contents were generally high during the night, resulting in high rates of respiration. Daytime net CO2 uptake was restricted by low light in the understory or by temporary reduction of thallus water content in the canopy. As a consequence, carbon balances were negative on many days. The results of these first field measurements of the CO2 gas exchange of a tropical lichen support the notion that the low abundance of macro lichens in tropical lowland forests is mainly caused by an unfavourable combination of high temperature and low light. Keywords: Leptogium azureum; lowland rain forest; carbon balance; lichen.