Diplodocus carnegii Hatcher, 1901, is a sauropod dinosaur that was originally recovered in the late 19^th century in the Upper Jurassic of North America. The large amount of bones recovered permitted the reconstruction of the original skeleton at the Carnegie Museum in Pittsburgh, Pennsylvania, United States of America. A series of casts of the specimen were made and donated by Andrew Carnegie to different countries in Europe and Latin America. The cast of D. carnegii mounted in 1912 at the Museo de La Plata was one of the nine replicas donated by Carnegie. The history of the discovery, the trip to Argentina by Carnegie Museum personnel, and the mounting of the cast skeleton are related in this contribution.