A female Blainville's beaked whale, Mesopiodon densirostris, 4.4m long was found stranded at 42km, South of Cassino, Rio Grande do Sul, Brazil. The triangular shape and dimensions of the pair of unerupted teeth (60 x 45 x 11 mm) in the lower jaw, together with the distance (180mm) from the alveoli to the hinder edge of the mandibular symphysis proved to be a good systematic character for females of this species. The vertebral formula is C 7 + D 10 + L 11 + Ca 15 (+2) =45. It presents 10 pairs of ribs, 6 being double headed. M. densirostris is the only species of Mesopiodon which seems normally to occur both north and south of the equator and also the onlv species of this genus to strand on oceanic island of tropical and warm-temperature waters. The present record is the first for the Soutn West Atlantic waters.