Samples collected from the Rosario Formation in northwestern Baja California provide faunas correlative with those reported from the middle to late Campanian. Two biostratigraphic zones previously established by Sliter (1968) are recognized primarily on the basis of 20 planktonic species and 21 selected benthonic species. The Maestrichtian Stage was not recognized within the area. This is due to the fact either that it is not represented in the strata or that the diagnostic planktonic species were environmentally restricted from this area. Foraminiferal faunas from northwestern Baja California can be successfully correlated with middle to late Campanian faunas from the Pacific Coast and Gulf Coast. Paleoecologic evidence suggests that foraminifera associated with Upper Cretaceous marine deposition in northwestern Baja California lived in a bathyal environment. The close proximity of these bathyal biofacies to basement outcrops indicates that a continental shelf was either very narrow or nonexistent in this area during the Upper Cretaceous. Planktonic evidence based on comparisons between Globotruncana and Globorotalia menardii and Globorotalia tumida suggests that water temperatures were warmer than they are at present. Lateral continuity of biofacies is generally good, but some lateral changes can be detected between lower and middle to upper bathyal biofacies.