The thallus structure of a common epiphytic lichen Hypogymnia physodes growingon the bark of Pinus sylvestris under a variety of microclimatic conditions was studied by means of fluorescence and scanning electron microscopy. The disparity between the thallus layer thicknesses of specimens growing in different habitats proved to be small though obvious, and differences also appeared in the amount of gelatinous substances in the hyphal cell walls ofthe cortex and algal layer. Furthermore, the occurrence of a pored epicortex of H. physodes at dry sites is reported. The size distribution of the thalli, thallus area and the production of soralia showed a complex interaction with microclimate. Against a background of the microclimatic data collected here, humidity is considered to be the most important factor affecting the anatomy and morphology of the thallus.