Waders inhabiting an area in which A. tyosenense is endemic were examined to determine if they serve as reservoir hosts for the parasite. Of the waders examined, 4 individuals from 2 species were found to be hosts for 18 to 1820 adult worms. Common and velvet scoters, which are known to be the final hosts of A. tyosenense, are rare in the infected areas, while waders are prevalent and prey on the bivalves known to be intermediate hosts of A. tyosenense. Thus, from the experimental results and circumstances presented herein, we conclude that waders are definitive and reservoir hosts of A. tyosenense in nature.