Ecological studies on water pollution in the Capibaribe-Mirim river were carried out during a period of 12 months (January to December 1974 covering the dry and wet seasons). Six collecting stations were established, from the upper course of the river, near Macaparana town, to the middle course, near Goiana town. In about 40 collected samples, 96 taxons, including species and varieties, were found. Reports and comments were made on the microflora and microfauna observed, more attention having been given to the Bacillariophyceae and Chlorophyceae present in the samples. The most frequent and dominant species of diatoms were the following: Biddulphia laevis (Ehr.) Hustedt, Synedra ulna (Nitzsch) Ehr., Eunotia pectinalis (Kutz.) Rabenhost, Nitzschia sigma (Kutz.) W. Smith, Navicula cuspidata var. ambigua (Ehr.) Cleve, Eunotia didyma Grunow, Amphora ovalis kutz., Amphora coffeaeformis Agard, Hantzschia amphioxys (Ehr.) Grunow, Nitzschia triblionella var. victoriae (H.) Grunow, Pinnularia acrospheria Breb., Pinnularia mesolepta (Ehr.) W. Smith, Rhopalodia gibberula (Ehr.) O. Müller, Surirella ovata Kutz. Table 1 shows the ecological inventory of each taxon, the halobic indices of the species, as well as the indicator species of the saprobic conditions of the water. Table 2 shows the frequency and distribution of diatoms in the several stations.