The concentrations of PCBs in sea water and blue mussels ranged from 0.5 to 4.0 ng · l^-1 (mean ± S.D.: 1.8 ± 0.6 ng · l^-1) and from 3.1 to 69.8 ng · g^-1 -wet (24.0 ± 19.0 ng · g^-1-wet), respectively, in the Seto Inland Sea. The main homologues of PCB in seawater and blue mussels were T₄CBs and H₆CBs, respectively, and their content ratios to total PCB were 40∼50%. Homologues with high chlorine content accumulated in blue mussels. By principal component analyses associated with the homologues of PCB in seawater, blue mussels and sediment, it was clearly shown that the sea area in Osaka Bay and Harima-nada were mainly contaminated with low-chlorine-content homologues of PCB which were mainly used in condenser and heating fluid, and that the other sea area was mainly contaminated with high-chlorine-content homologues which were mainly used in paints and varnish. More detailed analyses were possible by the principal component analyses with selected and important isomer data.