Field studies on the east side of Mount Katahdin, Maine, from 2001 to 2004, and an appraisal of published and unpublished Katahdin records from previous work documented 203 different bryophytes (131 mosses and 72 liverworts) for subalpine conifer forest, alpine tundra and cirque walls, and krummholz in between. This work represents the only recent investigation of a high altitude bryoflora in the mountains of the northeastern United States. Two mosses (Hygrohypnum smithii, Pohlia tundrae) are new to this region, and six others, including Neckera oligocarpa, are new records for Maine. Investigations of Grimmia (4 spp.), Cynodontium (2 spp.) and Kiaeria (2 spp.) clarified the application of taxonomic concepts for these mosses relative to previous published work. In conjuction with parallel studies of Katahdin lichens, eight habitats were specified for the study area. These habitats were largely the same places as those recognized for lichens. The eight habitats differed in the number of Arctic mosses and in unique occurrences of various mosses and liverworts. Mean Arctic-boreal-cool temperate values, introduced as an analytical tool to evaluate the distributional affinities of Kathadin lichens, were calculated for bryophytes for seven of eight habitats. The mean for each habitat class was well predicted by a multiple regression equation, with altitude, solar gain and snowpack persistence, but not substratum, as independent variables. Upper and lower altitutinal limits of bryophyte species occurrence were determined. The documented presence of Katahdin alpine bryophytes in four other mountain areas in northeastern United States and adjacent Québec, Canada, showed fewer of them in western areas (the high Adirondack Mountains, New York and Mt. Mansfield, Vermont), a possible outcome of increasing oceanic conditions eastward.