Corbiculae (Corbicula japonica prime) were found to be highly polluted with a herbicide, chlornitrofen (CNP), during May and June in paddy fields in Miyagi Prefecture. Analytical procedures for residual 1, 3, 6, 8- and 1, 3, 7, 9-TCDD in corbiculae were developed and the corbiculae were also shown to be contaminated with small amounts of 1, 3, 6, 8- and 1, 3, 7, 9-tetrachlorodibenzo-p-dioxin (TCDD), which are considered to be formed as impurities during the production process of CNP.Corbiculae were analyzed at regular intervals throughout a year at a fixed monitoring point in Miyagi Prefecture. The maximum concentrations of TCDD and CNP were 39ppb and 11.2ppm, respectively, in corbiculae on May 12, 1982, and then decreased rapidly after the end of the herbicide application period. The dissipation rate of TCDD was slower than that of CNP.An analytical procedure for TCDD based on dechlorination to unsubstituted dibenzo-p-dioxin with sodium borohydride in the presence of nickel chloride was also investigated. The dechlorination procedure can be used for the screening of TCDD in relatively highly contaminated samples, and gives the value of total TCDD.