Ecological theory suggests that sympatric species cannot occupy the same niche without exclusion of the weaker compet-

A new species of Mathevotaenia Akumyan, 1946 (Cestoda: Anoplocephalidae) is described from the Andean tuco-tuco, Ctenomys opimus Wagner 1848 (Rodentia: Ctenomyidae), collected in 1984 on the Altiplano of Bolivia. This is the second species of anoplocephalid cestode recorded from rodents of the genus Ctenomys, the first being Monoecocestus torresi Olsen 1976 from the Maule tuco-tuco, Ctenomys maulinus Philippi 1872, documented in southwestern Argentina. The new species of Mathevotaenia described here has more testes per segment than any other described species of the same genus from South America. The description of a new species from a decades-old specimen highlights the need for continuous examination of museum material as well as the need for collection of new specimens from rapidly disappearing biomes around the world.