A Gymnophallid cercaria was found in the gonadal tissue of Tapes philippinarum, and was described as a new species, Cercaria tapes n. sp. The host clams were collected from the intertidal zone of Seo-myon, Seo-chon gun, Korea from September 1980 to August 1981. The size of sporocysts was $380-650{ imes}130-160{mu}m$. The anterior part of the cercaria bears a subterminal opening and pharynx. The sporocyst bears ca. 30 cercariae and germ balls. The cercaria is elliptical in shape. The dimension of body is $160-187{ imes}62-77{mu}m$. the tail stem is $47-52{ imes}17{mu}m$, and the tail furcae is $67-72{ imes}12-15{mu}m$. Excretory bladder is Y-shaped, and the flame cell formula is 2(1)+(1)=4. The infection rate of the cercaria to the host clam was $2.01\%$ (42 infected/2,086 short-necked clams observed). The highest infection rate ($5.74\%$) was found in March, and the lowest one ($0.4\%$) observed in January. No infection was detected in May and June. The infection rates varied with the size of the short-necked clam. It was the clams of host of 2.1-3.0 cm, 3.1-4.0 cm and 4.1-5.0 cm in shell length are $0.38\%,;15.3\%$ and $0.09\%$ respectively. The cercaria was not observed in the specimens smaller than 2.0 cm in shell length.