Abstract Nearshore surface assemblages of phytoplankton and microzooplankton (mostly tintinnids) from water pumped continuously through a 53 μm net were collected along a 70 km longshore transect off southern California and a shorter offshore transect. The low biomass of the larger phytoplankton and the presence of many warm-water species was consistent with the prevailing El Nin˜o conditions. This was especially marked in the northern part of the longshore transect. To the south, assemblages were more “coastal” and there was a bloom of Stenosemella sp., the presence or absence of which was largely responsible for the pattern of the major microplankton assemblages, as established by dendrograms resulting from the use of the Bray-Curtis index for measuring similarity between samples. Patch length of assemblages was usually 10 km or less. The stormy conditions at the time of the cruise resulted in unusually high runoff from the coastal lagoons and were possibly related to the microplankton structure along the longshore transect.