Stems simple or branched, 70–160 cm; branches stiffly erect or arching-ascending, terete, faintly ridged, glabrous. Leaves strongly-ascending; blade linear-lanceolate, 10–50 x 1–3 mm, fleshy, margins entire, adaxial surface with sessile, dome-shaped hairs; axillary fascicles absent. Inflorescences racemes, elongate, flowers 1 or 2 per node, some flowers pseudoterminal; bracts both shorter and longer than, or longer than, pedicels. Pedicels strongly ascending, 5–25 mm, glabrous. Flowers: calyx narrowly campanulate, tube 4–6.5 mm, glabrous, lobes deltate, 0.5–0.9 mm; corolla pink, without 2 yellow lines and with dark pink spots in abaxial throat, 25–35 mm, throat pilose externally and villous within across bases and sinus of adaxial lobes, lobes: abaxial spreading, adaxial reflexed-spreading, 6–14 mm, pilose or sparsely so externally; proximal and distal anthers parallel to filaments, pollen sacs 2.8–5 mm; style exserted, 15–28 mm. Capsules subglobular to globular, 5–7 mm. Seeds dark brown to blackish, 1.1–1.5 mm. 2n = 28.
Phenology: Flowering late Aug–early Oct.
Habitat: Wet savannas and prairies, depressed areas of mesic savannas or open flatwoods, bogs, margins and centers of cypress domes, margins of ponds, lakes, brackish and freshwater marshes, open stream banks, roadsides, ditches.
Elevation: 0–100 m.
Ala., Del., Fla., Ga., La., Md., Miss., N.C., S.C., Va.
Agalinis linifolia is the only perennial member of the genus in North America. It and A. maritima are the only obligate wetland species of Agalinis in eastern North America; both grow in standing water.