Genetic variability was investigated in North American populations of the tree pathogenic fungi Nectria galligena and N. coccinea var. faginata. Genetic diversity was estimated by analysing RAPD and ribosomal DNA polymorphisms. The number of haplotypes was the maximum value in N. galligena (51), whereas 17 different haplotypes were detected among the 23 isolates of N. coccinea var. faginata studied. The frequency of polymorphic loci was greater in N. galligena (92%) than in N. coccinea var. faginata (35%). The estimate of genotypic diversity was a maximum in N. galligena while it represented only 59% of the maximum value for N. coccinea var. faginata. Gene diversity was also higher in the former (21%) than in the latter (6%). Moreover, four distance indexes calculated from the same genetic distance matrix gave similar results and showed that distances among isolates of N. galligena were four to nearly six times larger than distances among N. coccinea var. faginata isolates. An analysis of molecular variance based on genetic distances among haplotype frequencies confirmed that host origin (beech vs other species) was not the source of the genetic variability in N. galligena since less than 1% of the variability was attributable to the host factor. Overall, our results are consistent with the hypothesis that N. galligena is a pathogen native to North America while N. coccinea var. faginata may be a recent introduction.