Seasonal changes in phytoplankton populations in the chlorophyll-a concentration and on some environmental factors were studied monthly from June 1974 to September 1975 at Ubatuba, northern coast of São Paulo. Water temperatures ranged from 20ºC in August to 29.5ºC in March and salinities from 33.06 º/.. in March to 35.80 º/.. in January. The maximum salinity value observed in January near the bottom, associated with the thermal stratification found only during this month, seems to indicate an oceanic contribution to the enrichment of the local water mass. Pluviometric values ranged from 37.2 mm in August to 468.5 mm in January. Phytoplankton cell counts fluctuated from 64,000 to 1,028,000 cells/l and chlorophyll-a values varied from 0.52 to 6.86 µg/l. Maximum standing-stock was observed during summer, particularly in March, and it is probably associated with land drainage due to precipitation. A second bloom was observed in September 1975, and it is associated probably with the discontinuous impact of the wind. The phytoplankton populations were dominated by unidentified phytoflagellates, followed by diatoms and dinoflagellates. Nitzchia closterium and Thalassionema nitzschioides, along with Navicula sp., were the most representative species of diatoms. Among the dinoflagellates, Gymnodinium sp. was the most important. Blue-green algae and silicoflagellates were poorly represented. The diatoms Chaetoceros simplex, Chaetoceros spp, Nitzchia longissima, Nitzchia spp, Rhizosolenia delicatula and Rhizosolenia stolterfothi were found with high densities, only during bloom periods. The chlorophyll-a fractionation experiments demonstrated that organisms in size classe smaller than 20 µm represented, for all depths, between 54.93 and 98.15 º/.. of the total phytoplankton population.