Definitive non-avian dinosaur embryos, those contained inside fossil eggs, are rare(1,2). Here we describe the first known unequivocal embryonic remains of sauropod dinosaurs-the only known non-avian dinosaur embryos from Gondwana-from a nesting ground in the Upper Cretaceous stage of Patagonia, Argentina. At this new site, Auca Mahuevo (Fig. 1), thousands of eggs are distributed over an area greater than 1 km(2). The proportion of eggs containing embryonic remains is high : over a dozen in situ eggs and nearly 40 egg fragments encasing embryonic material were recovered. In addition to bone, these specimens contain large patches of fossil skin casts, the first definitive portions of integument ever reported for a non-avian dinosaur embryo. As morphology of the eggs enclosing these osseous and integumentary remains is identical, we propose that these specimens belong to the same sauropod species. This discovery allows the confident association of the megaloolithid type of dinosaur eggshell(3) with sauropod dinosaurs.