Calcareous fens are wetlands fed by calcareous groundwater and with predominantly low (< 1 m) vegetation. They are critical habitat for rare species, and development of tall vegetation alters this habitat. Our purpose was to better understand how human activities influence the fen habitat. In 1997 in eastern New York and northwestern Connecticut we located 25 fens using soil maps and field surveys, and delimited these sites using 73 indicator plants classified a priori as fen species, disturbance species, or non-fen woody species. In three plots in each fen we measured species cover, water table depth, organic matter, total inorganic nitrogen, groundwater pH and conductivity, and described land use on and within 100 m of each fen to identify the conditions that may lead to fen degradation. Total inorganic nitrogen correlated most significantly with plant species composition. Fen-species richness was negatively correlated with nitrogen, whereas non-fen woody species richness and cover were positively correlated with nitrogen, suggesting that nitrogen inputs contribute to dominance of tall vegetation in fens. Richness of disturbance indicator species was negatively correlated with soil organic matter. Thus, fens with mineral soil layers may be more susceptible to dominance by non-fen species. Percent cover of disturbance indicator species was greater in fens near public roads. Cover of Lythrum salicaria, an invasive weed, was positively correlated with land-use intensity and was greater near public roads. The use of soil maps, indicator plant richness and cover, inorganic nitrogen, water table, and soil organic matter may be an efficient method for locating, delimiting, and assessing the ecological status of fens.