Regional and local scale gradients controlling plant species distribution in mountain fens were studied in Yellowstone National Park, Wyoming, USA. Data on vascular and nonvascular plant cover, groundwater and soil chemistry, landforms, microtopography, and regional gradients of elevation, precipitation, and bedrock geology were collected and analyzed for 476 relevés from 166 fens. The pH of groundwater supporting fens ranged from 2.89 to 7.98. Six major bedrock types influence the chemical content of groundwater: three volcanic types, a glacial till complex containing sedimentary deposits, and two rock types altered by geothermal activity. Twenty-eight plant communities were identified through cluster analysis and table methods. Vegetation data were related to environmental gradients using DCA, CCA, and CCA with variance partitioning. The main environmental gradients affecting vascular plant species were site landform and stand topography, which separated fens formed in basins from sloping fens. Bryophytes were more strongly correlated with the acidity/alkalinity gradient of groundwater pH. For all species, the regional variables elevation, annual precipitation, and groundwater chemistry accounted for 40.7% of the total variation explained, while local variables site landform, stand topography and microtopography, and soil characteristics accounted for 43.9%.