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Closely spaced XBT and CTD data were used to study the sub-surface structure of features observed in satellite AVHRR sea-surface temperature (SST) images of the Gulf of California (GC), such as fronts, jets and gyres. The lowest SSTs in the GC are found around the midriff islands, and especially over the sills, because of strong mixing from tidal currents and breaking internal waves. This minimum SST area is limited to the south and north by sharp SST fronts, which frequently show convolutions, gyres, and filaments. A vertical section of temperature along the Gulf length shows that about 300 km south of the sills, the isotherms begin to spread out in the vertical, reducing stratification. The thermocline isotherms begin surfacing about 100 km south of the sill, forming the SST minimum area, which has a maximum temperature difference of ∼6 °C with waters to the south and a horizontal SST gradient of ∼0.05 °C km −1 . The evolution and subsurface structure of a cool filament, apparently originated as an instability of the SST front, was intensely sampled. Satellite images show that its head advanced due south at about 0.5 ms −1 , along the edge of a pool of warm surface water about 110 km in diameter. The thermocline below the warm pool was raised at its edge, and some isotherms reached the surface, producing the cool filament. The tilt of the temperature, salinity and density isolines at the edge of the warm pool produced a geostrophic jet, adjacent to the cool filament and just inside the warm pool, with maximum speed ∼0.5 ms −1 and extending down to ∼80 m. The filament lasted about 10 days, its width was ∼30 km and it reached a maximum length of ∼120 km.The volume transport of the jet in the top 100 m was around 0.7 Sv.