Pistia stratiotes L., a perennial water weed found in natural water bodies in Nigeria, removes organic and inorganic nutrients as it grows and spreads in the waters. In a lake which received a variety of pollutants, the plant reduced oxidizable organic matter (4-h permanganate value) by 61%, the biochemical oxygen demand by 85.3%, ammonia nitrogen by 95.3% and water soluble phosphorus by 85%. The plant also removed other mineral matter.