Herbs, annual, to 60 cm; rhizomes absent; stolons present; corms present. Leaves submersed, phyllodial, lenticular, to nearly terete, 12--53 ´ 0.3--0.7 cm; rare stranded plants without expanded leaf blades. Inflorescences racemes, of 2--7 whorls, floating or emersed, to 16 ´ 4 cm; peduncles 13.5--28 cm; bracts connate more than ¼ total length, ovate to lanceolate, 1.5--2 mm, delicate, not papillose; fruiting pedicels spreading to reflexed in flower and fruit, cylindric, 1.5--6.5 cm. Flowers 1.5--5 cm diam.; sepals spreading in staminate, appressed to spreading in flower and fruit in pistillate, often enclosing flower or fruiting head; filaments dilated, longer than anthers, glabrous; pistillate pedicellate, without ring of sterile stamens. Fruiting heads 0.4--0.6 cm diam; achenes oblanceoloid to obovoid, not abaxially keeled, 1.5 ´ 1 mm, beaked; faces not tuberculate, wings absent, glands absent; beak lateral, erect, 1.1 mm. Flowering summer--fall. Streams and lakes; 1500--2000 m; N.Mex.; c Mexico. Sagittaria demersa was known previously only from central Mexico. It is known in the United States from three recent collections taken in northern New Mexico.