Abstract
Infection of 11 species of larval trematodes in Semisulcospira libertina was examined in relation to distribution within the water system, host size, and frequency of multiple infection. Total prevalence did not have a fixewd tendency compared between upper and lower reaches within tributaries. Metagonimus yokogawai, Cercaria yoshidae and C. incerta were more prevalent in upper reaches. C. longicera and C. incerta infected in small size hosts while Cercaria sp. D (Opistorchiformes) infected in large size hosts. Multiple infection occured between M. yokogawai and C. introverta, and bewteen C. yoshidae and C. incerta in higher frequencies than those expected from random distribution. There is no species pair which overlapped in lower frequency than expected one.