Moss floras on granite outcrops in the Piedmont of Georgia and Alabama were studied by conducting thorough inventories of 15 outcrops, with the objective of investigating variation in moss flora composition among granite outcrops of different sizes. Across all 15 study sites 78 species were collected, with the number of species per site ranging from 23 at the smallest to 57 at the largest. Linear and nonlinear regression analyses comparing species number with area and with perimeter length showed that, of the four models tested, three nonlinear models produced substantially better results than the linear model, with the power model producing the best fitting curves among these three. In addition, species number was found to be more closely correlated with perimeter length than with area. Thirty-two of the most frequent species comprised the core group of granite outcrop mosses; they each occurred at 60–100% of the 15 sites and formed the bulk of the floras of all but the two largest outcrops. Eight species with primarily northern or southern distributions were found to also occur on granite outcrops, resulting in substantial range extensions. In the Piedmont these species occur only on granite outcrops, further emphasizing the uniqueness of these habitats in the southeastern U.S.A. Among these eight species were three new state records: Archidium minus and Dicranum polysetum in Alabama and Hygrohypnum alpestre in Georgia.