This study investigated the role of hydrology, soils, and trampling in determining the distribution of vegetation in an isolated wetland from 1996 to 1999. Grassy Pond, in Litchfield, New Hampshire, is a seasonally flooded basin marsh situated in sandy soils. It is a depressional wetland consisting of three connected basins; it has no surface-water inlets or outlets. This acidic, low-nutrient wetland contains several rare species and represents an uncommon ecosystem type in New England. A network of wells and piezometers monitored from 1996 to1999 established that the wetland receives an average of 95.4% of its growing season inputs from precipitation and the rest from shallow ground water flowing through the wetland; as a result, it experiences large fluctuations in water levels. Vegetation in the wetland fell into five major elevation zones. Variation in plant diversity within each zone, and differences between adjacent zones, result in part from differences in depth of organic layer, trampling by hikers, all-terrain vehicles, and native wildlife, and the extent of water-level fluctuations. Several of the low basin species are more likely to be found in trampled areas. Shrub invasion of the open basin areas is prevented by trampling in some areas and by high water levels in others. Variability in hydrology resulted in temporal, as well as spatial, variability in the plant community, as dry years yielded significantly greater diversity, including a large increase in tree seedlings. Both hydrologic variability and trampling are external factors that explain a significant portion of the variation in vegetation on a large scale and connect this geographically isolated wetland to the surrounding landscape. On a smaller scale, however, autogenic forces related to soil formation and plant species interactions may be more important in explaining the plant diversity in the wetland.