ONTOGENETIC VARIATION IN THE BONE HISTOLOGY OF CAYPULLISAURUS BONAPARTEI FERNÁNDEZ, 1997 (ICHTHYOSAURIA: OPHTHALMOSAURIDAE). In order to explore the osteohistological ontogenetic variation in ichthyosaurs, we studied the bone microstructure of the ophtalmosaurid Caypullisaurus bonapartei Fernandez. The analyzed thin sections were obtained from three ribs belonging to different individuals (MLP85-I-15-1, 83-XI-15-1 and 83-XI-16-1), from the Vaca Muerta Formation (Tithonian, Late Jurassic), in the Neuquén Basin (Argentina). The ontogenetic stages of these specimens have previously been determined based on the morphology of the humerus and the sclerotic rings. The first specimen is composed entirely of finely spongy tissue distributed around the medullary cavity. Microscopically, the spongy bone is secondary, with abundant inner spaces delimited by bone trabeculae. There are numerous overlapping generations of lamellar bone. The second specimen (a juvenile) shows primary spongy bone surrounding the medullary cavity and Sharpey's fibers. The third specimen lacks a medullary cavity; instead, the medullary region is occupied by secondary spongy bone, uniformly distributed through the entire section. These results suggest that the macroscopic changes classically attributable to ontogeny have a correlation in bone microstructure. The bones of immature individuals show some primary bone, while the bones of the mature specimens are characterized by the exclusive presence of secondary bone. These features are easily identifiable, and provide an auxiliary criterion for the determination of ontogenetic stages in incomplete and fragmentary specimens.