During the summer of 1984, nannoplankton dominated the phytoplankton assemblages of the coastal upwelling zone off San Quintin bay (30¿24'N, 116¿00'W), Baja California, Mexico. Upwelling was masked by the warming of surface and near-surface waters as a product of El Ni¿o influence. Diatom and dinoflagellate abundances were between 1 and 2 orders of magnitude lower than those reported for the summers of 1977 and 1979. A 21-day time series shows that assimilation numbers of surface phytoplankton had a range of 2.0 to 5.4 mg C mg chlorophyll a-1 h-1. These values are lower than those reported for coastal waters of the California Current for 1969, 1970, 1977, and 1979 and are similar to values reported for 1983. In 1984, surface chlorophyll a concentrations and primary productivity were an order of magnitude lower than values reported for 1977 and 1979. ¿ American Geophysical Union 1987