The complete life cycle of Echinostoma audyi, a new species with 37 collar spines, has been completed experimentally. There are at least three redial generations. Lymnaea rubiginosa (Michelin) serves as the first intermediate host. The same and other freshwater snails such as Gyraulus convexius- culus (Hutton), Indoplanorbis exustus (Deshayes), Pila scutata (Mousson), and Bellamya ingallsiana (Lea) serve as second intermediate hosts. Adult worms are obtained under experimental conditions from ducklings; pigeons; little cuckoo doves, Macropygi7 ruficeps (Timminck); black-headed or chestnut munias, Lonchura ferruginosa (Sparrman); spotted munias, Lonchura punctulata (Linnaeus); and Java sparrows, Padda oryzivora (Linnaeus); but not from rats, mice, and hamsters. Eggs are passed 8 days after infection. The adult worm resembles Echinostoma revolutum (Froelich, 1802), Looss, 1899; E. paraulum Dietz, 1909; and E. lindoense Sandground and Bonne, 1940; but the cercaria shows marked morphological differences. This study shows that the taxonomic position of closely related echinostomes cannot always be established satisfactorily unless cercarial characteristics are also known. The freshwater snail Lymnaea rubiginosa (Michelin) collected from ponds in the sur- roundings of Kuala Lumpur and from paddy fields in Kuala Pilah (60 miles southeast of Kuala Lumpur) was often found infected with an echinostome cercaria with 37 collar spines and fin folds on the tail. Metacercarial cysts found in the same and other freshwater snails developed into adult worms when fed to pigeons, ducklings, and various birds. The complete life cycle has been worked out under experimental conditions. The study of the adult worms and the various larval stages re- vealed an undescribed species of the genus Echinostoma Rudolphi, 1809.