The samples of water collected from the rhizosphere of Pistia plants in the lake, from the feeder streams and the root washings were examined for different pathogenic and other microorganisms. The results (Table 6) indicate that while the streams feeding the lake were free from any pathogenic organisms at the origin, the water samples at Zones A, B and C, and the Awba stream at the point of entry, revealed the presence of Balantidium coli. Amoebic trophozoites and cysts, and helminthic larvae. The root washings also showed small numbers of rotifers, Euglena sp., Colpidium sp., Paramoecium sp., and very few Vortieella sp. This also indicates that the root system, by its nature, harbours many microorganisms.