Upper-ocean eddies are commonly observed from remote sensing, but submerged eddies are more difficult to detect. During July 2004, a 21-day hydrographic survey in the southern region of the California Current was carried out to investigate the mesoscale variability. We observed for the first time a subsurface anticyclonic eddy off northern Baja California with the same water mass characteristics as the California Undercurrent. The core of the eddy was quasi-circular with radii of 35 km and thickness of 250 m. The maximum swirl velocity was ∼3 cms−1. The water mass of the core of the eddy was characterized by potential temperature of 11°C, salinity of 34.5, and dissolved oxygen of 1.4 mll−1. The eddy propagated westward. The subsurface warm-eddy could transport relatively saline water into the North Pacific subtropical gyre.