In order to investigate bioaccumulation of mercury in Iranian coastal waters fish, tiger tooth croaker (Otolithes ruber) and yellowspotted trevally (Carangoides fulvoguttatus) from the Gulf of Oman and grass carp (Ctenopharyngodon idella) and silver carp (Hypophthalmichthys molitrix) from Zabol Chahnimeh reservoirs were sampled in May 2012. In these ecosystems, total mercury concentrations were measured in muscle and gill of these four fish species. Both regions did not reveal a significant difference. In H. molitrix and O. ruber, mercury concentrations in gill increased with increasing length (H. molitrix, r = 0.94, p < 0.005; O. ruber, r = 0.94, p > 0.005) and weight (H. molitrix, r = 0.94, p < 0.005; O. ruber, r = 0.82, p > 0.04). Generally, mercury levels in the analysed fish samples were higher than WHO, FDA and US-EPA legal limits.