Plants usually rooted in soil. Sterile leaves lanceolate to lance-ovate to ovate to deltate or cordate. Petiole of sterile leaf 1--31 cm, not inflated. Blade of sterile leaf 1--3-pinnate, 2--41 × 2--20 cm; segments lobed or incised, elliptic to lanceolate to ovate or deltate, to 12.5 cm; proximal pinnae ± alternate. Fertile leaves lanceolate to ovate to deltate or cordate, 2--117 × 2--48 cm. Petiole of fertile leaf 1--46 cm. Blade of fertile leaf 3--4-pinnate proximally, 2-pinnate distally; terminal segments linear. Sporangia usually crowded between segment midvein and revolute margin, with 13--71 indurate annulus cells. Spores 32 per sporangium, 96--124 µm diam. 2 n = 154, 156.

Aquatic to semiaquatic in swamps, bogs, canals, ponds, lakes, ditches, marshes; 0--200 m; Calif., Fla., La., Tex.; worldwide in tropical areas except Africa.

Ceratopteris thalictroides is common in Florida but rare elsewhere. It is tetraploid ( n = 77, 78), the two cytotypes reproductively isolated. It can be distinguished from the diploid C . richardii on the basis of spore number per sporangium. The single population in southern California may have been a recent introduction and apparently has not persisted. Several populations are of hybrid origin, with reduced spore viability and irregular meiotic pairing. These include some in southern Florida and Texas.