The epiphytic lichen flora on basal trunks of old Salix caprea and Populus tremula trees was surveyed in eight old-growth stands in the southern and middle boreal forests of Finland. Both the total species number and average species number per tree were higher on S. caprea than on P. tremula. The epiphyte flora on S. caprea was dominated by widespread generalist foliose lichens, but some rarer specialists were also found. The flora of P. tremula was quite unique, and was dominated by crustose lichens and bryophytes, foliose and fruticose lichens being only sparsely represented. Cyanobacterial lichens, such as Lobaria spp., Nephroma spp. and Peltigera spp., were frequent on both tree species. Structure and acidity of the bark, as well as the availability of different microhabitats, are suggested to be the most important factors contributing to floristic differences and diversity. The bark of S. caprea and P. tremula is one of the key habitats for conservation of epiphyte diversity in the boreal forests of Finland.