One hundred twenty-seven species of Upper Cretaceous Foraminiferida were recovered from a sandstone-siltstone sequence in Letterbox Canyon southeast of Carlsbad, San Diego County, California. This rich fauna offers a new evaluation to the Campanian-Maestrichtian boundary in the sedimentary rocks of the Southern California Coastal Plain. These Upper Cretaceous sedimentary rocks are temporarily assigned to the Rosario Formation of Baja California, Mexico. In Letterbox Canyon, they lie nonconformably on the Lower Cretaceous Green Valley Tonalite of the crystalline basement complex and are overlain unconformably by the sandstones of the Eocene Delmar Formation. Two distinct faunules are established from the Rosario beds. The Frondicularia goldfussi faunule which is late Campanian in age, and the Nodosarella gracillima faunule which is early Maestrichtian. The Nodosarella gracillima faunule is correlated with the Globotruncana fornicata-Globotruncana tricarinata assemblagezone of the Navesink Formation of the New Jersey coastal plain. It is also correlated with the uppermost beds of the Navarro group from the Gulf coastal area and with formations from Australia, Tunisia, Puerto Rico, and Trinidad B.W.I. The basin of deposition is thought to have been topographically restricted. The samples taken appear to represent sites located approximately 4,000 feet off shore on a basin floor sloping 5º or less. During the Upper Campanian the basin is considered to have been located on the inner continental shelf with waters of normal marine salinity having surface temperatures of 20° centigrade, and depths of 50-80 meters. During the Lower Maestrichtian the basin was located on the central continental shelf with deeper (80-100 meters) and colder (16º centigrade) waters of normal marine salinity.