The Louisiana quillwort (Isoetes louisianensis) is a small, grass-like aquatic plant of the family Isoetaceae. It is "one of the rarest quillworts in North America." It occurs in only five locations in St. Tammany and Washington Parishes of Louisiana and some spots in southern Mississippi (United States). It is federally listed as an endangered species, partly due to its highly restricted range. Distinctive characteristics of this species include brown-spotted sporangial walls and megaspores with highly reticulate ridges, producing a spiny effect. The leaves are up to 40 cm long.