Genetic diversity in eight populations of Sphagnum capillfolium from different Bulgarian mountains was investigated by means of isozyme electrophoresis. High levels of allelic diversity were found (H-s = 0.119), comparable to earlier estimates for northern European populations (H-s = 0.116). Strong differentiation among populations and a low number of widespread genotypes suggest a high degree of isolation and restricted gene flow between populations, which is consistent with generally small and scattered populations. The large proportion of distinguishable genotypes (mean 0.498) suggests high levels of out-crossing either currently or in the past. Introgression between S. capillfolium and S. rubellum, a species not found in Bulgaria, was suggested by the occurrence of rubellum-alleles in five populations from different mountains. This could be explained by an ancient hybridization event in a sympatric population. Based on (1) the high genetic diversity, (2) the fairly wide distribution of alien alleles, and (3) the isolated distribution of populations even within one mountain. a possible survival of S. capillifolium in the Balkan area during the Quaternary ice periods is hypothesized.