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Plant communities of the fog-free coastal desert of northern Chile depend on highly variable rainfall inputs. Precipitation is concentrated in the winter, bur it varies in quantity and frequency among and within years. We discuss the importance of this variability in the structure of the plant communities along a latitudinal gradient encompassing tbe entire range of the coastal desert (27-30 degrees S). We present the results of three years of records of plant cover, species richness and phenology in permanent plots, and laboratory essays to determine the moisture thresholds for seedling emergence from soil samples. The communities comprise a high proportion (30-60%) of annuals. Northwards, shrubs increase in relative importance. Geophytes are mole abundant in the center of the gradient. In a wet year, absolute plant cover varied between 10 and 60% among sites, more than one half of which wag contributed by ephemeral plants. In a dry year (< 20 mm of rain) the ephemeral plant cover did not develop. This minimum amount of rai required for ephemeral plant growth coincides with the threshold for seedling emergence determined in laboratory experiments. Flowering showed a pronounced peak two months after the rain, and in some species it may continue during part of tho dry period. Maximum richness of native plant species and high endemism occur in the central part of the gradient studied, where areas protected from human activities are lacking.