A population of the Asiatic clam,Corbicula fluminea, and eight species of cohabiting unionid mollusks were examined for the presence of the oligochaete,Chaetogaster limnaei. The mollusks were collected in navigation Pool 19, upper Mississippi River. Infestation ofC. limnaeiin the Asiatic clam was significantly greater than in unionids. A decline in occurrence and abundance ofC. limnaeiinfesting the Asiatic clam was found in the winter when the Asiatic clam population also declined. Of the unionid mollusks,Leptodea fragilishad the highest rate of infestation and the highest number ofC. flumineaper individual.