Summary Fog formation occurs frequently along the coast of nothern Chile. The fog envelopes the slopes and the summit of the coastal ranges. Water condenses in this zone and effects the development of an usually lush vegetation known as a fog oasis in this arid or semi-arid environment. In the Fray Jorge National Park, the fog-dependent vegetation consists of evergreen forests which are surrounded by thorn scrub and succulents. Lichens play an important role in these habitats. They cover the phanerophytes of the different zones of the coastal area as epiphytic vegetation and have both high biomass and species diversity. The objective of the first part of this work was a description of the epiphytic lichen vegetation for a selected, representative area of about 10 km2 of the Fray Jorge National Park and to study the environmental relationships of these lichens with special emphasis on their water relations. Redon (1982) described 54 different epiphytic lichen species for this area. A phytosociological study resulted in the definition of 2 lichen-associations which are characterized by species combinations, dominant life forms, and species diversity. The Oropogonetum loxensis lichen community grows on shrubs and trees in the zone which is directly affected by persistent fog. On the other hand, the Ramalinetum cactacearum lichen community covers the shrubs in the region below the fog belt. Along a transect which streched from the drier inland depression without fog influence to the top of the coastal range where there is regular fog formation, the changes in the lichen vegetation and the gradation between the 2 communities were characterized. At the habitats of the Oropogonetum and the Ramalinetum, diurnal courses of light intensity, air temperature, air humidity and lichen thallus temperature were measured during characteristic wheather conditions in the different seasons throughout the year. Lichen water content of typical, selected species was followed at the same time by weighing of samples. The species of the Oropogonetum are often thoroughly moistened during the night by condensing fog which forms droplets on the thalli. In the early morning hours, maximum water contents of the lichens up to 153 and 172 % (by dry weight) respectively were measured. After disappearance of the fog during the late morning hours, the thalli increasingly dry out as temperatures increase and air humidity is decreased. Thallus water content reaches a minimum of about 16 % in the early afternoon. In contrast, water relations of the species of the Ramalinetum are much more unfavourable. Moistening by liquid water seems to be restricted to very infrequent dew condensation and to the few rain events in winter (situations, which were not observed during this study in the field). The habitats of the Ramalinetum are not reached by the fog. However, fog formation in the higher altitudes of the coastal ranges is correlated with a substantial increase in air humidity in the depression where the Ramalinetum occurs. The lichen thalli are able to absorb this water vapor from the air even though no condensation occurs. Thus, in the early morning hours after nights with high air humidity, hydration of the species of the Ramalinetum increases, and water contents up to about 37 % were measurable. Subsequently the thalli dry out again and reach their minimum water content after noon, similar to the lichens of the Oropogonetum. Water exchange between lichens and air seems to take place so quickly, that the thalli are almost in equilibrium with the water potential of the ambient air, at least during the time periods of low solar radiation before noon. The same is the case for the species of the Oropogonetum on days without fog formation but with high air humidity. In a second communication of this series, the importance of the habitat conditions for the photo-synthetic primary production of the lichens will be assessed.