Gaseous diffusion resistances for carbon dioxide and water vapour, thallus water content and thallus water potential were experimentally determined on species of the Stictaceae. The diffusion resistance to water loss was high only at low water contents and correlated closely with thallus water potential. Carbon dioxide diffusion resistances, however, were high at both low and high water contents and, even at medium water contents, were still an order of magnitude greater than the water resistance. These results indicate that carbon dioxide and water vapour exchange occur by different pathways in these lichens. Consequently it is suggested that the lichens have structural adaptations which separate the functions of water uptake, water storage and carbon dioxide exchange.