ABSTRACT: The Jurassic-Cretaceous boundary is the most problematic of all Phanerozoic system boundaries. Its definition is obscured by almost imperceptible faunal and floral change, nomenclatural problems resulting from the poor definition of its bounding stages, the over reliance of calpionellids for correlation, and the provinciality of ammonite zones in the European type area. The unfortunate decision of ammonite stratigraphers to use Calpionella Zone B to mark the base of the Berriasian set back the resolution of the Jurassic-Cretaceous boundary problem by thirty-five years in Europe, North America, and elsewhere. In this report a more detailed radiolarian zonation for the Upper Jurassic is introduced. The radiolarian biostratigraphy has been integrated with that of the North American ammonite, Buchia, calcareous nannofossils, and calpionellids as well as with new and existing U-Pb geochronometry. Investigations focused on uppermost Jurassic to lowermost Cretaceous successions in central Mexico, Baja California, California, and the West Indies have recognized four new subzones in both radiolarian Zones 2 and 4. Pillow basalt at La DÃ©sirade dated at 143.734ma Â± 0.060ma [0.042%] (U-Pb zircon age on coeval plagiogranite) is intercalated with red ribbon chert containing an upper Subzone 4 beta2 radiolarian assemblage with corporeal taxon Neovallupus spp. This horizon likewise occurs in Mexico, where it can be directly related to ammonite-bearing strata occurring slightly below the boundary between Imlay’s Kossmatia-Durangites and Substeueroceras- Proniceras assemblages, and can be recognized in Argentina and Antarctica as well. The composite data from the North American record indicate that the European calpionellid biozones are diachronous between Europe and the Western Hemisphere. The boundary between Zone 4, Subzone 4 alpha2 and Zone 5, Subzone 5A, which corresponds to the traditional Jurassic-Cretaceous boundary, is well represented at Grindstone Creek in the California Coast Ranges in the upper part of the Buchia sp. aff. B. okensis Zone. In view of our findings, however, it also may be desirable to consider two other alternatives for the placing the Jurassic-Cretaceous boundary: one at the base of new Radiolarian Subzone 4 alpha1 and the ammonite Substeuerocas-Proniceras assemblage, and the other at the base of the Valanginian. The traditional boundary horizon as well as the two alternatives can all be related to new and existing U-Pb geochronometric data. Nomenclatural problems surrounding formational units in the Mexican Upper Jurassic have been addressed. Four new members for the La Caja Formation; two new members for the Taman Formation; and one new formational unit, the Santa Rosa Formation have been introduced.