Bryant’s Bog consists of two concentric zones, a free-floating and a grounded mat surrounding a central pool. The vegetation of the two zones differed in species composition, biomass and extent of tree mortality from 1969-1975. The grounded mat had less biomass and more tree mortality. Midsummer values in 1974 and 1975 of 17 chemical and physical parameters of shallow groundwater (15-30 cm below the surface) were not significantly different (P < 0.05) between the two zones or the two summers. The shallow groundwater had the low Ca + Mg concentrations characteristic of ombrotrophic bogs, moderately acid pH values characteristic of weakly minerotrophic bogs, and moderate concentrations of K, P and N. The Ca + Mg and pH values were comparable to those in six other bogs in the area while K, P and N concentrations were noticeably higher in Bryant’s Bog. The pattern of tree mortality was consistent with changes in water levels in lakes and streams in the region. Depth from the surface to the water level was significantly greater on the free than the grounded mat. Variations in water levels are the most probable cause of tree mortality and differences in vegetation of the two zones.