The histology of the long bone of a prosauropod dinosaur is described. The bone is in very good condition and is still birefringent like recent bone. The bone has a laminar histological pattern, quite unlike that of recent reptiles, but very like that of recent artiodactyls. The vascularization of the dinosaur bone is compared quantitatively with that of recent mammals and reptiles. The amount of vascularization is much greater than in recent reptiles, and is of the same order as, but rather more than, in recent mammals. It is argued that this may indicate physiological specialization in the dinosaurs.