In June 1987 the geostrophic flow in the coastal transition zone off northern California (between 50 and 150 km off the coast from Point Reyes to just north of Cape Mendocino) was dominated by a well-defined, southward-meandering current. Three vertical sections are presented that show the hydrographic structure of the current down to 100 m and its relationship to the distribution of phytoplankton biomass. The sections show that the geostrophic adjustment brought cold, saline, deep water up to the surface on the low steric height, or cold side of the flow, and that this upwelled water supported a relatively large diatom biomass (chrolophyll-a concentrations between 1 and 10 mg m-3). We present particle size spectra and photomicrographs of the phytoplankton that show that the diatom biomass was dominated by chain-forming species (e.g., Skeletonema costatum, Chaetoceros spp., Thalassiosira spp., and Rhizosolenia alata, but also a single-celled Actinocyclus sp.). Photosynthetic light response measurements reveal that these diatom communities were capable of high photosynthetic rates (Pmax between 5 and 25 mg C mg Chlα-1 h-1). Although most of the diatoms were located in cold, slow-moving water on the low side of the current, some were being carried downstream. High chlorophyll concentrations were observed at depths >75 m in and along the cold edge of the flow in all of our sections; we show evidence that in two out of three cases this was the result of water mass subduction. ¿ American Geophysical Union 1991