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In this study phytoplankton were used as a water quality index in agricultural sewage for feasibility study of aquaculture in Ahwaz and Khorramshahr region. Monthly sampling was done in 6 stations during one year period (2008-2009). 34 phytoplankton genuses were identified. The most frequent classes were Bacillariophyceae (diatoms), Cyanophyceae, Chlorophyceae, Dinophyceae. The water flow of drainage channel had inverse relationship with phytoplankton density. Diatoms were present in all stations. Palmer index suggested that station 2,5,6,3,4 had the highest rate of pollution respectively and station 1 was relatively polluted. The stations of 1,2,5,3,6,4 had the highest value of useful phytoplankton index for fish growth.