Rhizomes 1-3 mm thick, nodes with whorls of roots. Culms 5-80 cm, erect, glabrous. Ligules of lower leaves 0.4-3 mm; ligules of flag leaves 1-4(5.5) mm, usually lacerate; blades 1-13 cm long, 1-4 mm wide. Panicles 2.5-18 cm long, 1-6 cm wide. Spikelets 4-8.5(9) mm; rachilla internodes 1-1.5 mm. Glumes 4-8.5(9) mm; lemmas 3-6.5 mm; paleas 2.8-6 mm; anthers 1.5-3.5 mm. Caryopses 1.5-3 mm. 2n = 42, 44, 66, 84, 88, 132.

Dupontia fisheri grows in wet meadows, wet tundra, marshes, and along streams and the edges of lagoons, ponds, and lake shores, in sand, silt, clay, and moss, rarely in bogs. It is sometimes dominant, or co-dominant, with Alopecurus borealis.

Two subspecies are sometimes recognized in North America. Dupontia fisheri R. Br. subsp. fisheri supposedly differs from subsp. psilosantha (Rupr.) Hultén in being less than 40 cm tall, having erect panicle branches, 2-4 florets per spikelet, pubescent, obtuse lemmas, and 2n = 84, 88, or 132. Dupontia fisheri subsp. psilosantha is taller, has reflexed panicle branches, 1-2 florets per spikelet, more or less glabrous, acute lemmas, and 2n = 42 or 44. Plants referable to subsp. psilosantha are restricted to coastal marshes, rarely penetrating inland along riparian habitats, from James Bay to the lower arctic archipelago. Plants referable to subsp. fisheri are less halophytic and more northerly in their distribution, being found in a variety of inland marshes and wet tundra habitats from northern Alaska to Ellesmere Island. Intermediates are readily found (e.g., hexaploid, 2n = 66 hybrids from Alaska) and the correlations among chromosome number, morphology, ecology, and distribution are relatively weak in North America and Greenland. For these reasons, no subspecific taxa are recognized in this treatment.