The life history of a trematod fluck, Himasthla kusasigi was studied on the morphological characters of metacercariae, its infection rate in the clam and the contamination experiment to the herring gull, Larus crassiostris. The size of the metacercariae was $210{sim}230{ imes}220{mu}m$, excysted metacercariae was $420{ imes}160{mu}m$ and it had 31 collar spines. The ventral sucker($112{ imes}100{mu}m$) was located just below the central part of the body. Esophagus was narrow and long, the intestine was branched in the upper part of the ventral sucker and extended to the hind part of the body. Excretory bladder without the infected part was located in the hind part of the body. The branched excretory tube was filled with the small granules and extended to the pharynx area. The infection rate of the metacercariae varied from place to place. In Naecho do and Puan the rates were $98.4\%$ and $95.9\%$ respectively. The infected number of Himasthla kusasigi per clam was positively proportional to the size of the clam. The adult fluckes developed from the metacercariae were obtained from the experimented herring gull. The metacercariae emerge from their cysts in the stomach of the herring gull, then they migrate it the intestine and 25 days after, most of them were found in the intestine. The size of the adult was $4.8{sim}7.5{ imes}0.22{sim}0.24mm$, oral sucker; $68{sim}120{ imes}70{sim}120{mu}m$, pharynx: $80{ imes}50{mu}m$, ventral sucker: $630{sim}680{ imes}610{sim}680{mu}m$. The head collar $48{sim}75{ imes}10{mu}m$ had 31 collar spines. The isolated trematod fluck from the samples was classified as a Himasthla kusasigi by the morphological characteristics of the adult flock and its metacercariae. We oberved that the second intermediate host of Himasthla kusasigi was hard clam, Meretrix lusoria, while its final host was herring gull, Larus crassiostris and Tringa ochropus.