We calculated the mass balance of major congeners of dioxins derived from chloronitrophen (CNP) and pentachlorophenol (PCP) found in Sendai Bay, in order to characterize the present status of marine pollution caused by dioxins and enable the establishment of appropriate countermeasures against dioxin contamination. According to published statistics, about 58 000 t of CNP and about 22 000 t of PCP were shipped to Miyagi Prefecture over a 40-year period, and based on the these amounts of CNP and PCP, we estimated that 30.7 t of 1,3,6,8-tetrachlorinated dibenzo-p-dioxin (1,3,6,8-TeCDD), 1,3,7,9-TeCDD, and octa-chlorodibenzo-p-dioxin (OCDD) were introduced into the terrestrial environment as impurities contained in both pesticides. Mass balance analysis results show that most of these dioxins remained in the terrestrial environment and were not transported by runoff into Sendai Bay. Moreover, degradation mainly reduced the amount of dioxins in the terrestrial environment, instead of runoff or volatilization. Although large amounts of CNP and PCP were shipped to Miyagi Prefecture, only 0.8% of the total quantity of 1,3,6,8-TeCDD, 1,3,7,9-TeCDD, and OCDD contained in pesticides that were applied to paddy fields reached Sendai Bay via the discharge of major rivers. Moreover, most dioxins transported to the bay either flowed offshore or settled at the bottom of the bay. And there was very little bioaccumulation in marine organisms, particularly commercially valuable species.