Pesticides are used extensively in agricultural lands in Northern provinces of Iran which makes river ecosystems more vulnerable to pesticides pollution. This study was conducted in three river estuaries (Babolrud, Tajanrud and Gorganrud), focusing mainly on the concentration levels of organophosphate pesticides (diazinon, malathion, azinphos-methyl) in the muscle tissue of golden mullet fish, an economically important and highly consumed fish in the region. The research was conducted in 2011-12. During the study, 27 samples of mullet fish were collected. After extraction and purification process, the average level of concentrations of diazinon, malathion in mullet fish muscle tissue were measured using Gas Chromatography. Based on the results, the highest average concentrations of azinphos-methyl belonged to samples taken from Babolrud river estuary (1.28 mg/kg), while these concentrations for Tajan and Gorganrud samples were respectively 0.76 and 0.04 mg/kg. The highest concentration of malathion in mullet fish muscle was found in the samples taken from Tajan River (0.19 mg/kg). The concentration of malathion in samples taken from Babolrud and Gorganrud rivers were also 0.17 and 0.07 mg/kg, respectively. The highest concentration of diazinon in mullet fish muscle was found in samples taken from Babolroud River (0.27 mg/kg), and then in samples from Tajan and Gorganroud rivers (respectively 0.09 and 0.08 mg/kg). It was concluded that the most contaminated river estuary was Babolrud river estuary.