The roles of Pronodularia japanensis and Corbicula spp. were studied experimentally to examine the cause of the decrease in Tanakia tanago in the Handa Miyakotanago protection area, Ootawara City, Tochigi Prefecture, central Japan. Tanakia tanago showed a clear preference for Pronodularia japanensis, while it rarely used Corbicula spp. as a host. Embryos were found at greater frequencies in the gills of Pronodularia japanensis collected from the Kuji River than in those collected from a small creek at Handa. The survival rate of embryos was also lower in the latter. These results suggest that the poor condition of mussels, due to water pollution in the Handa Miyakotanago protection area, was the reason for the decline in the population of Tanakia tanago in the late 1990s.