A sensitive and rapid procedure is described for the simultaneous measurement of mean rates of net CO2-exchange and nitrogenase activity (acetylene reduction) in cyanophilic lichens. It involves a reaction gas chromatographic technique for the determination of CO2 and ethylene. Data on concurrent rates of net CO2-exchange and nitrogenase activity in Peltigera canina (L.) Willd. var. praetextata (Florke em. Sommerf.) Hue and Stereocaulon paschale (L.) Hoffm. at different levels of thallus water content are presented in order to illustrate the application of the new method.