Satellite thermal imagery and in situ biological and physical data are presented that describe the spatial variability of phytoplankton biomass and species composition in relation to the physical structure at the sea surface during persistent upwelling off northern California. Surface patterns in temperature, geostrophic velocity, chlorophyll, and particle size structure show that the coastal zone was dominated by two water masses separated by a well-defined physical boundary. This boundary is apparent in satellite thermal imagery as a cold front and in dynamic height as a meandering jet in the California Current. On the landward side of the front (jet) we observed cold, coastal water that was relatively enriched in phytoplankton biomass, due to the presence of large diatoms. On the seaward side of the front (jet) we observed relatively warm water where the phytoplankton biomass was low and the diatoms were largely absent. Broad tongues and narrow filaments of cold, chlorophyll-rich water that extended over 100 km offshore were present in our study area. These features were bounded by the meandering coastal jet. The coldest, most chlorophyll-rich water in these tongues and filaments was located between the onshore and offshore flows in relatively slow moving water, but entrainment of coastal water into the jet was apparent. ¿ American Geophysical Union 1990