A new species of Notocotylus was found parasiting a freshwater pulmonate snail, Biomphalaria peregrina. Naturally infected snails were collected from two temporary ponds in the Nahuel Huapí National Park in Patagonia. The characteristics of the larval stages are presented. Experimental adults were recovered from the intestinal caeca of ducks and chicks. Adults of Notocotylus biomphalariae n. sp. exhibit an aspinose tegument, two lateral rows of 11 ventral glands and a median row of four, a uterus with 12-16 coils of which 2-4 are previtelline, a metraterm equivalent in size to 65-68% of the cirrus-sac length, a previtelline field which extends to the middle of the body, a lobed testis and a genital pore closely posterior to the intestinal bifurcation. The rediae have one to three cercariae. The cercariae, when shed, are trioculate and have a long tail; they encyst in the environment and become infective 12 days after encystment.