The serpentine-substrate effect is well documented for vascular plants, but the literature for bryophytes is limited. The majority of literature on bryophytes in extreme geoedaphic habitats focuses on the use of species as bioindicators of industrial pollution. Few attempts have been made to characterize bryophyte floras on serpentine soils derived from peridotite and other ultramafic rocks. This paper compares the bryophyte floras of both a peridotite and a granite outcrop from the Deer Isles, Hancock County, Maine, and examines tissue elemental concentrations for select species from both sites. Fifty-five species were found, 43 on serpentine, 26 on granite. Fourteen species were shared in common. Twelve species are reported for the first time from serpentine soils. Tissue analyses indicated significantly higher Mg, Ni, and Cr concentrations and significantly lower Ca∶Mg ratios for serpentine mosses compared to those from granite. Soil analyses demonstrated significant differences between the two substrates.