A cotylocercous cercaria occurring in the marine snail, Astraea americana, at Tortugas, Florida, was found to encyst in small fish as second intermediate hosts.Fish experimentally infested with the cysts were fed to the gray snapper, Neomaenis griseus, and adult worms developed in the intestine and pyloric caeca which were identified as Hamacreadium mutabile Linton, 1910, a member of the sub-family Allocreadiinae.A general relationship of the cotylocercous cercariae to the family Allocreadiidae is discussed.