A new method has been developed to measure chlorophyll a fluorescence of bryophytes and lichens in situ in the field. Specially designed aluminium stands fixed to rocks or other suitable substrata allow several cryptogam thalli to be measured sequentially with only one pulse amplitude modulated (PAM) chlorophyll a fluorescence analyser. The stands guarantee a fixed position for the fluorescence probe in relation to the thallus surface and also provide the possibility to darken the thalli so that both the apparent and the potential quantumefficiency of PSII (▵F/Fm′ and Fv/Fm) can be measured. A detailed analysis of chlorophyll a quenching parameters (qP, qN, NPQ) can also be performed with only minimal changes in the environmental conditions. The method allows easy detection of periods of metabolic activity and inactivity of the poikilohydric organisms. In combination with gas–exchange and microclimatic measurements the carbon balance also can be calculated. The method is described in detail and examples of its application in the field are presented