One of the problems for rivers in Iran is related to dispersal of the waste effluents from trout farms to them and Shemrood River is one of them, a center for trout farming in Guilan province which includes four main active parts. Thus in order to evaluate the effects of trout farm effluents on the river, the macrobenthos communities of selected parts of the river was analyzed. Eight stations mainly located near the wastewater inlets and outlets were selected then sampling was done on macrozoobenthos communities, they were identified to family or genus level, counted and evaluated by ecologic and biological indices. Results show that thirty-four taxa groups of benthic samples such as worms, mollusks and different orders of aquatic insects have been observed by an average number about 12.65 ± 4.77 numbers per square meters. Also mean amounts for BMWP, ASPT and the Hilsenhoff Index (HFBI) were determined afterwards. BMWP index was acceptable for outlet stations but the amounts of EPT% and EPT/Chiro had been decreased in these stations. In addition benthic biomass has been increased after the outlets. In conclusion, it can be said that the effect of dispersal of the sewage from trout farms on Shemrood River is not significant and indicates the ability of the river for self-purification.