A portable system for CO2 gas exchange measurements is described which allows determination of photosynthesis and respiration rates of lichens (and mosses) in the field under light, humidity and temperature conditions which are close to ambient. Time periods for enclosure of the thalli in a cuvette of this CO2-porometer for gas exchange measurement are shortened to between 1 and 2 min, so that an experimental influence on their water relations is small. Between the periods of gas exchange measurement, the lichens experience their natural microclimate conditions. Thus, measurements approximate closely the true gas exchange rates of the lichens under natural conditions. Diurnal courses of CO2 exchange of lichens found growing in a Mediterranean habitat in the spring are demonstrated, as determined by this method.