The elementary structural units where changes of plant communities come about are synusiae and studies of successional processes induced by external factors should focus on that scale. Synusia is a structural part of a plant community inhabiting a special microhabitat, with a specific floristic composition and consisting of species that belong to the same stratum and that do not differ fundamentally in either periodicity or way of exploitation of their environment. We studied the responses of the moss synusiae to the cement kiln dust pollution in Myrtillus site-type forests in the vicinity of the Kunda cement plant (North Estonia). The synusiae were clustered into eight societies. The species content of the bryophyte synusiae changed completely along the alkaline pollution gradient. Synusiae of Dicranum polysetum—Pleurozium schreberi, Aulacomnium palustre—Hylocomium splendens and Ptilium crista-castrensis—Hylocomium splendens societies, characteristic of unpolluted forests, were replaced gradually by the synusiae of Rhytidiadelphus squarrosus—Hylocomium splendens, Climacium dendroides—Hylocomium splendens and Rhodobryum roseum—Rhytidiadelphus triquetrus societies as pollution increased. Brachythecium rutabulum—Rhytidiadelphus triquetrus and Fissidens dubius—Rhytidiadelphus triquetrus societies were indicative of forests with heavy alkaline pollution. The composition of epiphytic bryophytes also changed almost completely along the pollution gradient; Fissidens adianthoides, Tortula ruralis and Barbula unguiculata were indicative of the heavily-polluted zone. We conclude that alkaline pollution has a clearly detectable impact on the forests' bryophyte synusiae in terms of both species composition and typological structure.