The contamination with norovirus (NV), a causative agent of gastroenteritis, in wild Pacific oysters Crassostrea gigas and Mediterranean blue mussels Mytilus galloprovincialis was surveyed from 2005 to 2006 by RT-PCR, collecting monthly 10 ∼ 20 samples for each species in Shiogama Bay (Shiogama Port), northeastern Japan. The bivalves examined were highly contaminated with NVs, especially with Genogroup 2 (G2), in winter and early spring, with the peak of 91% (G2) for oysters in April and 74% (G2) for mussels in March. The contamination rates in cultured Pacific oysters, concomitantly collected in a different bay (Onagawa Bay) in the same period of time, were much lower than those in wild oysters. When the contamination rates were compared among wild oysters collected at three sites with different distances from a sewage treatment plant, the incidence of G1 genogroup of NVs was higher in the oysters collected at sites nearer to the treatment plant, indicating that the drainage from the plant is the major virus source in this area. An improvement of virus inactivation in the treatment process is believed to be necessary to prevent viral contamination in coastal populations of bivalves.