1 - The quantitative relation between net phytoplankton, nanoplankton and zooplankton during the high and low tide for five days at a fixed station is reported here. The samples were collected at surface, 2 and 4 m near the Baguassu river, a typical region of mangrove environment. 2 - Measurements were made of transparence, salinity, temperature, pH, dissolved oxygen, suspended matter and dry weight (inorganic + organic). Culture of bacteria was also carried out to determine the degree of contamination of the water. 3 - Total phytoplankton cell counts were lower during high tide showing a good inverse correlation to salinity. 4 - The total organic matter varied from 6.80 to 16.80 mg/l and the dry organic matter of net plankton varied from 0.084 to 3.64 mg/m³. These results are presumably due to a great portion of the particulate matter that may be originated from nanoplankton and fine detritus. 5 - The average of nanoplankton was 97% greater than net phytoplankton and the net phytoplankton was in average 99,7% greater than zooplankton.