Satellite-derived time series for the California Current System (CCS) showed marked changes in the surface chlorophyll a concentration (Chl, mg m-3) associated with the 1997--98 El Ni¿o. In addition to the previously known decrease in Chl off Southern California (Fiedler, 1984), we also observed a significant increase of Chl off Baja California. Whereas the extent of eutrophic (Chl>1.0) areas decreased throughout the CCS, the extent of mesotrophic areas (0.2<Chl<1.0) off Baja California approximately doubled. The reduced area of eutrophic waters is attributed to weakened upwelling but the increase in the offshore mesotrophic area off Baja may be caused by blooms of nitrogen-fixing cyano-bacteria. Using revised Coastal Zone Color Scanner data we detected similar changes during the 1982--83 El Ni¿o. ¿ 2000 American Geophysical Union