Sphagnum and Sphagnum-dominated peatlands have long provided a model for analyses of community structure, and recent work has elucidated the genetic structure of various Sphagnum species in the boreal zone. We report here analyses of genetic variation and population differentiation in a tropical species, S. tumidulum, from Réunion Island. Forty-eight plants (including one sample from Madagascar) were genotyped at 10 microsatellite loci. The species is highly variable on Réunion Island and approximately 22% of the total variation is partitioned among populations. Thirty-one multilocus genotypes could be distinguished and clones represented by more than one gametophyte stem were restricted to individual populations. Our results suggest the occurrence of sexual reproduction on Réunion Island, although sporophytes have not been observed there. Gene flow among most populations appears to be very limited.