Crustaceans are ecologically important, because of their effective role in food web and community structure of ecosystems. Taxonomic identity of fresh water crabs has been studied in Iran representing Potamidae family as dominant fluvial crabs. In 2001 Khatami recorded an unknown oligochaete in the mantle cavity of fresh water crab Potamon persicum. The present study further separated and identified an oligochaete from Potamon persicum in Jajrood River, east of Tehran, Iran. Two hundred and fifteen specimens of Potamon persicum were collected using a trap during four seasons from Jajrood River (51° 41´ to 51° 48´N and 35° 37´ to 35° 47´E) during March to August 2004. The specimens were taken to the research laboratory, dissected after biometry
and the mantle and branchial cavities were examined for the existence of oligochaetes
