Submersed aquatic perennials of fresh water, rooting or drifting; dioecious. Stems simple or dichotomously branched. Leaves numerous, sessile, whorled, margins minutely toothed. Flowers unisexual, with slender pedicel-like perianth-tubes from sessile axillary spathes; sepals 3, green, herbaceous; petals 3, white, membranous, > sepals. Male flowers 2-4 from each spathe, each with 9 stamens; filaments glandular-papillose; female flowers with 3 staminodia, stigmas 3, bifid; all flowers with central nectary. Fruit obscurely trigonous, 3-6-seeded. Spp. 2, of subtropical America. Adventive sp. 1.