This research was done in Tonekabon River in 1996. The results indicated that the Tonekabon river fishes belong to 5 families as Salmonidae, Cyprinidae, Athenrinidae, Cobitidae and Gobiidae. The most frequency (89.4 %) and distribution of Salmonidae situated in upstream of the rivers belong to salmo trutta fario. From Cyprinidae, Alburnoides bipunctaus with 26.4%, Capoeta capotea with 19.9% and Barbus lacerta with 5.9% had the maximum frequency and distribution along the rivers comparing to other species. From Gobiidae, Neogobius melanostomus with 8.3% and from Cobitidae, Cobitis taenia with 7.3% had the maximum frequency. The amount of variety and accurence on species were increasing from up to the end of the river and the minimum was zero and the maximum were 1.997 and 2.173 in river basin respectively.