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A new species of Didymium (Myxomycetes), D. operculatum, is described in this paper, and details of its life cycle are provided. The new species was recorded during studies of the Atacama Desert in Chile. It has been collected directly in the field and isolated in moist chamber cultures prepared with material from an endemic cactus. The distinguishing characters of this species are its dehiscence by means of an apical operculum combined with a whitish calcareous stalk and the banded reticulate ornamentation of the spores. The morphology of this new myxomycete was examined with scanning electron microscopy and light microscopy, and micrographs of relevant details are included in this paper. Some comments are made on the patterns of distribution of Didymium species in a lands and adaptive characters enabling this genus to colonize such extreme environments. It is proposed that a longer cycle and the ability to resort to resistant forms many times during their development reflect the response of these myxomycetes to the largely unfavorable conditions of their environment.