[in German wit English summary: ] Colonization of Polytrichum, Dawsonia and allied genera (Polytrichales, Musci acrocarpi) by fungi is a very common, or in some host species, even universal phenomenon. Presently there are more than 40 ascomycetes known to be (obligately) associated with the gametophytic stage of polytrichaceous mosses. They represent a heterogenous assemblage regarding their biological nature (saprophytic, parasitic, and gall-inducing species) and systematic position (members of at least six orders). All known species are listed, their hosts (about 20 species in all), with the infected organs indicated. Several morphological adaptations to the unique substratum are discussed. It is noteworthy that the microhabitats of the plants are often occupied by different specific fungi. The spaces between the leaf lamellae provide the most favourable niche. 21 ascomycetes have been demonstrated to inhabit these interspaces apparently without causing any symptoms. They offer striking examples of convergent evolution, e. g. extremely minute and reduced ascocarps.