Abstract: In this study concentrations of Parathion (organophosphorous pesticides) was determined in four commercially valuable fish species, Rutilus frisikutum, Clupeonella delicatula, Mugila auratus and Vimba vimba, from four sites of Tajan River in July 2004. The concentration of parathion was measured by gas chromatography electron-capture detection (GC-ECD). Concentrations of the parathion in examined fish species ranged from 5.94 (site III) to 49.57 (site IV) lg kg-1 (wet weight). No difference found between kind of fishes in each sites about parathion concentrations but there are two groups of sites that were significantly different from one another in terms of parathion concentrations: sites 2, 3 and 4 < site 1 (p<0.05). Parathion in the edible parts of the investigated fish were in the permissible safety levels for human uses and should not be public concern among peoples in Mazandaran province.