The bryophyte communities on glacial erratic boulders in the Adirondack Mountains of New York are used as an experimental system to test the predictions of island biogeographic theory. Moss species composition, abundance, richness, and diversity were determined for 39 boulders. Bryophyte species richness on these insular habitats ranges from 3 to 14. Competitive dominance of the component moss species was determined. Boulders were characterized as to size, degree of isolation, and microhabitat heterogeneity. Patterns of disturbance and gap formation in the moss cover, and patterns of recolonization were quantified. The distribution of bryophyte species among individual boulders does not conform to the predictions of island biogeographic theory. Neither the size of the boulder nor its degree of isolation are significantly related to species richness. Richness was also found to be independent of the abundance of the competitive dominants. Overall measures of microhabitat heterogeneity were not correlated with species richness, although a weak relationship with microtopographic diversity was detected. We suggest that the high frequency of gap formation on these boulders maintains the community in a non-equilibrium state. The observed patterns of species richness may be the result of population level processes that govern dispersal and establishment among boulders and among gaps on the boulder surface.