The hepatopancreases of freshwater crabs (Eriocheir japonicus) collected from three Japanese rivers (Barato, Shiribetsu, and Tone) were analyzed for planar halogenated aromatic hydrocarbons (HAHs), including polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and coplanar polychlorinated biphenyls (PCBs). The hepatic glutathione S-transferase (GST)dependent enzyme activities in the crab hepatopancreas were also measured to examine their potential as biomarkers for the contaminants. Crabs from the Tone River, which runs through industrial, agricultural, and urban areas, have the highest concentrations of HAHs (4,100 pg/g fat weight), followed by those from the Barato River (2,430-2,970 pg/g fat weight), whereas crabs from the Shiribetsu River were relatively less contaminated (1,350-1,800 pg/g fat weight). Identification of numerous PCDD and PCDF congeners in crabs from all three rivers provided evidence that one of the major sources of PCDDs and PCDFs was waste incineration. In addition, crabs from the Barato and Shiribetsu Rivers were notably contaminated with 1,3,6,8- and 1,3,7,9-TeCDD congeners, which suggests that a possible source was chlornitrofen, which has been extensively used in paddy fields as a herbicide. Calculation of 2,3,7,8-tetrachlorinated dibenzo-p-dioxin toxic equivalents (TEQs) showed that the causal contaminants of higher TEQs in crabs from the Tone River (94.7 TEQ picograms per gram fat weight) were PCDDs and PCDFs, although the most important contributor to the total TEQs was coplanar PCBs (49.95%). The crab hepatopancreas appeared to have abilities to transfer glutathione to 1-chloro-2,4-nitrobenzene (CDNB) and 3,4-dichloronitrobenzene (DCNB). The crabs with the highest TEQ levels showed the highest GST activities. The current results and our previous data lead us to conclude that cytochrome P450 and GST-dependent enzyme activities (benzo[a]pyrene hydroxylation and CDNB and DCNB conjugation) in freshwater crab hepatopancreases are likely to be useful biomarkers for the contamination of planar aromatic hydrocarbons such as benzo[a]pyrene, PCDDs, PCDFs, and coplanar PCBs.