This paper reports a quali - and quantitative study on phytoplankton occuring in the estuary of Igarassu river (Lat. 7º 49'S-34º 52'W), one of the most important in the Itamaracd region (50 km north of Recife, PE, Brazil). Samples were taken during the period May, 1973 to May, 1974. Forty two species of diatoms were identified and listed, representing the most frequent group of the marine microalgae collected. Special emphasis is given to the ecological conditions of the estuary.