BONE HISTOLOGY OF THE TITANOSAUR BONITASAURA SALGADOI (DINOSAURIA: SAUROPODA) FROM THE UPPER CRETACEOUS OF PATAGONIA. The bone histology analysis of the titanosaur Bonitasaura salgadoi, from Río Negro Province, Argentina, was analysed in detail. It represents the first complete histological characterization of a South American sauropod based on a single specimen. Multiple samples were taken from different parts of the skeleton according to available fragmentary elements as well as to their paleohistological importance. The femur and humerus are characterized by a wide cortex mainly composed of primary, highly vascularized fibrolamellar tissue, interrupted by at least one growth mark. None of the samples show external avascular lamellar tissue (EFS). Samples from the ulna, one metacarpal and a caudal vertebral zygapophysis show histological similarities, allowing recognition of great internal remodeling with several generations of secondary tissue. Although very reduced, the primary fibrolamellar tissue was recognized with a LAG or annuli. In contrast, samples of dorsal ribs show a significant intraelemental variability. Only in proximal sections the fibrolamellar tissue has a notorious banding of longitudinal vascular canals, accompanied by at least two LAGs and a line of resorption. These features show that differential modeling process acted in the construction of these complex osseous elements. Finally, the recognized microstructural features allow the assignation of this specimen to a HOS (Histological Ontogenetic Stage) 9. The comparison with other known sauropod growth curves also allows inferring the adult femur length of this species reached up to 1.8–1.9 m.