Lysimachia filifolia, a member of the Primulaceae (primrose) family, is a small shrub 15 to 50 cm (0.5 to 1.6 ft) tall. The linear leaves measure 1.5 to 5.4 cm (0.6 to 2.1 in) long and 0.3 to 1.8 mm (0.01 to 0.07 in) wide and are usually alternately arranged. They are single-veined and sparsely hairy or hairless. The bell-shaped flowers are reddish purple, 6 to 10 mm (0.2 to 0.4 in) long and borne singly on flower stalks about 18 to 30 mm (0.7 to 1.2 in) long that elongate upon fruiting. Fruits are thick, hard capsules about 5 to 6 mm (0.2 in) long that contain numerous minute, nearly black, irregularly shaped seeds.