A pair of high-resolution oceanographic surveys in March and April 1995 revealed a large and rapid transition from late winter to spring conditions in the coastal zone off central and southern California. These data are unique in capturing the detailed three-dimensional physical structure of and biological response to the spring transition in the southern California Current System (CCS). Changes associated with the transition included a strong tilting of isopycnals, which lifted by up to 60 m near the coast and dropped 20--40 m offshore, a subsequent increase in cross-shore density gradients, the development of a strong nearshore equatorward jet, and an increase in net equatorward transport from the shelf break out to 300 km offshore. The most dramatic physical changes were confined to the shoreward 150 km and extended at least to the depth of the core of the California Undercurrent (~300 m). In response to these physical changes, there was an apparent strong increase in primary productivity, as indicated by changes in nearshore vertically integrated fluorescence and beam attenuation coefficient. Atmospheric and oceanic conditions in the CCS were near seasonal norms in the winter and spring of 1995, implying that a transition of the magnitude and rapidity observed here may be an annual event. Furthermore, the development of the coastal upwelling jet was independent of the winter manifestation of the main core of the California Current, which was maintained well off shore. This suggests that the California Current is regenerated seasonally through the development and offshore evolution of the coastal upwelling jet. It is not known whether the new jet joins and strengthens or replaces the offshore core of the previous winter.