This paper presents the occurrence of 25 moss species on trunks of alien trees in Wrocław (without city forests), their species diversity, frequency, coverage in the plots, and preferences to the host-tree species. In general, 36% of the trees colonised by bryophytes in the city were of non-indigenous origin (11 taxa), and the most frequently occupied were Populus xcanadensis Moench., Acer negundo L. and Robinia pseudoacacia L. Research showed that alien trees hosted a large group of species rarely noted in the city, such as: Syntrichia virescens (De Not.) Ochyra, Orthodicranum tauricum (Sapjegin) Smirnova, Leskea polycarpa Hedw., Hypnum pallescens (Hedw.) P. Beauv. and Orthotrichum affine Schrad. ex Brid. But the most total coverage in the plots was occupied by two facultative epiphytes, Orthotrichum diaphanum Schrad. and Hypnum cupressiforme Hedw. Comparing the results of this study with data presenting bryoflora of Robinia pseudoacacia L. and Quercus rubra L. found in secondary woodlands, it appears that these phorophytes in the city are colonised by different bryophytes. Results of UPGMA classification of bryofloristic similarity of all phorophytes in the city are discussed in reference to the theory that alien species can facilitate the occurrence of native species, as formulated by Rodriguez (2006).