A highly anomalous water mass of apparent subarctic origin, characterized by negative temperature and salinity anomalies, high concentrations of dissolved oxygen and inorganic nutrients, and unusually high chlorophyll fluorescence, was observed off southern California during summer 2002. This water mass was confined spatially to a narrow ribbon in the upper pycnocline: at a density of σt = 25.0 kg m-3 and ~150--350 km offshore, within and seaward of the main California Current core. These observations are very similar to those made off the Oregon and Vancouver Island coasts at about the same time, implying a displacement of subarctic waters more than 1200 km equatorward within the California Current. The maintenance of strong water property anomalies over this distance demonstrates the efficacy of the California Current as a conduit of water masses and plankton. The magnitude of the anomalies observed in July 2002 suggest the occurrence of the largest subarctic intrusion into the California Current System in the 50+ year historical record and may reflect low-frequency climate variability.