Physcomitrium pyriforme (Funariaceae) is a monoicous moss with the potential for producing sporophytes either via outcrossing or intra-gametophytic self-fertilization. A core set of microsatellite markers was identified for use in population genetic studies of this species, and employed to ascertain its mating patterns. An initial collection of 88 sporophytes gathered from widely separated locations in a meadow in central Ohio, U.S.A. was screened for genetic uniformity based on trnL-F DNA sequences, and found to display considerable heterogeneity. Fifty-three members of the largest clearly defined clade were selected for genotyping at 6 variable microsatellite loci having expected heterozygosities ranging from 0.11 to 0.70. Fifty-two individuals (98.2%) were homozygous at all 6 loci, strongly indicative of self-fertilization. Only one individual (1.8%) appeared to be an outcrossed sporophyte, being heterozygous at 4 loci. Twenty-three samples that constituted a second well-marked clade displayed complex microsatellite genotypes strongly suggestive of a polyploid cytotype that, although not readily amenable to further analysis, is also not inconsistent with the predominantly selfing mating pattern exhibited by the 53 others. Male and female branches within gametophyte stems were observed to develop simultaneously or nearly so, and in 4 dense moss clusters within which samples of 8 gametophytes were genotyped, all 8 were found to be identical within each cluster. These results indicate that the overall mating pattern is predominantly selfing, and suggest that the species reproduces principally in what is, in effect, a clonal fashion, via the union of genetically identical gametes. This may be occurring both within and between gametophytes.