North European epiphytic lichens are often genetically impoverished compared with their North American counterparts. This has been hypothesized to impede sexual reproduction due to reduced chances of finding compatible mating type partners. We compared genetic variation and reproductive mode in two threatened Scandinavian lichens, Evernia divaricata and Usnea longissima, with more viable populations in North America to see (i) if these species also show genetical depletion in northern Europe and (ii) if the occurrence of sexual propagules (ascospores in apothecia) is more prevalent in genetically diverse populations. Genetic variation of the fungal component was assessed by sequencing two nuclear rDNA gene regions (ITS andIGS) in 1005 and 1477 thalli, collected from 92 and 160 localities of E. divaricata and U. longissima, respectively. Scandinavian populations of both species were almost devoid of genetic variation compared with much higher genetic diversity in North America. We found no support for the proposed relationship between genetic diversity and fertility. Fertile thalli were found in several genetically invariable populations. Fertility increased with population size and regional abundance in E. divaricata, but not in U. longissima. In Scandinavia, E. divaricata was more fertile than previously recorded, whereas all sampled populations of U. longissima were sterile and possibly clonal. Key words: allelic richness, dispersal, epiphytic lichens, gene diversity, old-growth forests. Les lichens epiphytes europeens sont souvent appauvris genetiquement, comparativement a leurs contreparties americaines. On a formule l'hypothese que ceci nuirait a la reproduction sexuelle, du a la reduction des chances de trouver un partenaire de type de croisement compatible. Les auteurs ont compare la variation genetique et le mode reproducteur chez deux lichens scandinaves menaces, l'Evernia divaricata et l'Usnea longissima, avec des populations plus viables de l'Amerique du Nord afin de verifier (i) si ces especes montrent egalement un epuisement genetique dans le nord de l'Europe et (ii) si la presence de propagules sexuelles (ascospores dans des apotheces) prevaut plus fortement dans des populations genetiquement diverses. Les auteurs ont evalue la variation genetique de la composante fongique en sequencant deux regions du gene nucleique tADN (ITS et IGS) chez 1005 et 1477 thalles recoltes a partir de 92 et 160 localites, pour l'E. divaricata et l'U. longissima, respectivement. Les populations scandinaves des deux especes sont pratiquement depourvues de variation genetique comparativement a une diversite genetique tres fortement marquee en Amerique du Nord. Il n'ont trouve aucun support pour la proposition suggerant une relation entre la diversite genetique et la fertilite. On a retrouve des thalles fertiles dans plusieurs populations genetiquement invariables. La fertilite augmente avec la dimension de la population chez l'E. divaricate, mais non chez l'U. longisima.En Scandinavie, l'E. divaricata s'avere plus fertile que reconnu jusqu'ici, alors que toute les populations echantillonnees de l'U. longissima se sont averees steriles et possiblement clonales. [Traduit par la Redaction] Mots-cles: richesse allelique, dispersion, lichens epiphytes, diversite genetique, forets surannees., Introduction Lichens are symbiotic associations of heterotrophic fungi and photosynthetic green algae and (or) cyanobacteria. They represent an important part of the biodiversity in terrestrial arctic and boreal ecosystems, being [...]