Two new species of fossil Marsupialia are described from beds of late Tertiary age on the Altiplano south of La Paz, Bolivia. Borhyaenidium altiplanicus n. sp. (Borhyaenidae, Hathlyacyninae) is erected on a specimen from the Mauri VI "Formation" of Huayquerian (late Miocene) age near Achiri. B. altiplanicus is the most generalized of known species of Borhyaenidium and differs little from its probably Santacrucian (early Miocene) ancestor Perathereutes pungens Ameghino, 1891. Borhyaenidium was previously known only from beds of Huayquerian and Montehermosan (Pliocene) age in Argentina. Sparassocynus heterotopicus n. sp. (Didelphidae, Sparassocyninae) is based on a specimen from the Umala Formation of Montehermosan age near Umala. S. heterotopicus is the most generalized species of Sparassocynus known, and is the first species known outside of Argentina, where the genus is recorded from beds of Huayquerian through Uquian (early Pleistocene) age.--Journal abstract.