The epiphytic bryoflora growing on trunks of Quercus pyrenaica Willd. and Q. robur L. has been surveyed throughout an extended area in the northwest of the Iberian Peninsula. Seventy four species are catalogued (14 liverworts and 60 mosses) with an indication of their distribution in the area. Knowledge of the regional flora is supplemented with a total of 44 novelties for 7 of the 8 provinces considered. Substrate affinity for several facultative epiphytes is discussed on the basis of their frequency on bark. The distribution of regionally rare species such as Harpalejeunea molleri, Metzgeria fruticulosa, M. temperata, Orthotrichum consimile, O. scanicum, O. shawii, Ulota coarctata and Zygodon viridissimus is used in conjunction with overall species richness and the degree to which they are representative of the regional flora to assess the local significance of the different localities and to select a list of hotspots for conservation.