Annuals [perennials]. Stems prostrate, 15–60 cm, hairy, sometimes sparsely so, glabrescent. Leaves hairy; blade base broadly cuneate, margins entire, apex ± rounded. Pedicels 9–15 mm. Flowers: sepals 5, ovate to oblong, calyx bilaterally symmetric; corolla white with yellow throat, 5–10 mm; stamens 2–4, equal. 2n = 56.
Phenology: Flowering May–Nov.
Habitat: In water or on mud in lakes, ponds, pools, in and around marshes and ditches, ephemeral pools on rock outcrops.
Elevation: 0–2300 m.
Alta., Sask., Ala., Ariz., Ark., Calif., Colo., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Md., Minn., Miss., Mo., Mont., Nebr., N.Mex., N.C., N.Dak., Okla., S.Dak., Tenn., Tex., Va., Wis., Wyo., introduced in Europe (Greece, Spain).
Bacopa rotundifolia is the most widespread member of the genus in the United States; it is invasive in rice fields.