The MD 55 cruise of the R/V Marion Dufresne was an important French-Brazilian expedition devoted to the exploration of the shallow offshore waters and benthos off the south-eastern coast of Brazil. Four polyplacophoran species belonging to the families Ischnochitonidae Dall, 1889 and Acanthochitonidae Pilsbry, 1893 were sampled during the cruise and are studied herein. We compare the material with the original descriptions and voucher material whenever possible. Range extensions are reported for Stenoplax marcusi (Righi, 1971), Acanthochitona ciroi Righi, 1971, and A. terezae Guerra-Junior, 1983. A new species of Acanthochitona Gray, 1821 is described, Acanthochitona oxum n. sp. It differs from other Western Atlantic congeners by the presence of round to oval tegmental pustules and the straight external margin of the insertion plate.