An interval of reversed magnetic polarity has been located in Upper Cretaceous rocks of the Great Valley Sequence of California. The interval has been found at four sites, two on each side of the Sacramento Valley. In each case, the reversed interval is associated with the same ammonite local range zone. Firm biostratigraphic correlation and the structure of the magnetic polarity time scale allow us to determine that the reversed interval corresponds to marine magnetic anomaly 33–34 and represents the lower part of the Campanian Stage. The reversed interval serves as a marker horizon linking the different faunal assemblages and sedimentary facies of the Great Valley Sequence. It also provides the first direct correlation of provincial biostratigraphic zones of the North Pacific province with the Cretaceous stages in Europe.