The investigation of mollusks and the swimming-out of pre-larva fish from mussels in the Lake Uiam was carried out from March to November in 1981. The results of these investigations were summarized as follows: 1. The dominant species of mollusks living in the Lake Uiam is Corbicula fluminea. 2. The size of Semisulcospira gottschei and Corbicula fluminea is larger in the southern parts of the lake and gets smaller toward the north of the lake. 3. Semisulcospira gottschei and Corbicula fluminea are collected from all areas in the lake, regardless to the soil texture and water depth. 4. The number of species of mollusks in the lake is two classes, three orders, seven familes, including sixteen species. 5. The swimming-out rates of pre-larva fish are 43.9%, 4.5%, and 1.9% in Anodonta fukudai, Lanceolaria acrorhyncha, and Unio douglasiae respectively. 6. The authors confirmed one more mussel(Lanceolaria acrorhyncha) from which pre-lava fish have swum out. 7. The swimming-out of pre-larva fish from mussels takes more than a month.