Orygmatobothrium schmittii n. sp. collected from the spiral valve of Mustelus schmitti in the southwestern Atlantic Ocean at the Mar del Plata coastal region, Argentina, is described. The new species differs from O. musteli (Van Beneden, 1850) as described by Euzet (1959) in the size of body, scolex, gravid proglottis, cirrus sac and eggs; in the structure of eggs; in neck and cirrus length; in cirrus shape and by testes number. The cestode described here is different from O. zschokkei Woodland, 1927, and O. plicatum Yamaguti, 1934, in the location of the genital pore and in having bothridia with smooth margins respectively. Anatomical differences with the cestode described by Yoshida (1917) as O. velamentum are listed. The cestode parasitic of the same host in the same region and mentioned as O. velamentum by Ostrowski de Nuñez (1973) is probably cospecific with O. schmittii n. sp.
Plathelminthes, Cestoda, Eucestoda, Tetraphyllidea, Phyllobothriidae, Orygmatobothrium, Pisces, Triakidae, Mustelus, Argentine, Atlantic coast, parasitology, new species.