Ten enzyme systems coding for 13 putative loci were studied in 12 European species of Sphagnum sect. Acutifolia (peat mosses). On average seven plants from each of 73 collections made throughout N Europe were analysed. Despite complex morphological variation, all species expressed unique allele combinations. Principal components and cluster analyses based on allele frequencies showed that sect. Acutifolia can be divided into two major groups; one comprising Sphagnum fimbriatum, S. girgensohnii and S. molle; the second comprising S. capillifolium, S. angermanicum, S. fuscum, S. quinquefa- rium, S. rubellum, S. subfulvum, S. subnitens and S. warnstorfii. These groups are linked by S. russowii, which is shown to be an allopolyploid, with fixed heterozygosity at 6 to 8 of the loci. The presumed progenitors of S. russowii are S. girgensohnii and S. rubellum. Allelic data also indicate that S. teres from sect. Squarrosa is related to S. girgensohnii and S. fimbriatum. The bisexual species tend to be less variable than the unisexual species.