The author analysed the microflorafrom three ponds situated in the Cactarium from the Botanical Garden of Rio de Janeiro, Guanabara. Pond nº 1 - This pond showed a very intensive development of the microflora in the coldermonths. The diatoms were very frequent. Those diatoms that indicate saprobic conditions were the following; Gomphonema gracile Ehr., Pinnularia maior (Kutz) Cleve, Gomphonema parvulum (Kutz) Grunow, Navicula mutica Kutz., Pinnularia borealis Ehr., Pinnularia microstauron, (Ehr) Cleve, Pinnularia acrospheria Breb., Hantzschia amphioxys (Ehr) Grunow, Nitzschia palea Kutz., Eunotia pectinalis (Kutz) Rabenh. Pond nº 2 - In this pond we found diatoms that indicate polluted waters as follows; Eunotia pectinalis (Kutz) Rabenh., Gomphonema parvulum (Kutz) Grunow, Hantzschia amphioxys (Ehr) Navicula mutica Kutz., Pinnularia borealis Ehr., Pinnularia maior (Kutz) Cleve, Pinnularia microstauron (Ehr) Cleve. Pond nº 3 - The frequence of the microflora was rather reduced. Comparing the pond nº 1 with the one nº 2, the diatoms and chlorophyceae decreased in the hotter months. The oligosaprobic species found were the fallowing: Eutonia pectinalis (Kutz) Rabenh., Gomphonema gracile Ehr., Gomphonema parvulum (Kutz) Grunow, Hantzschia amphoxys (Ehr) Grunow, Pinnularia borealis Ehr., Pinnularia maior (Kutz) Cleve. Considerations were also made to the chlorophyceae as to saprobic regimen of the material estudied. Reference is made in the text to ecological details and comments. Eunotia angusta f. crenulata Elever-Euler and Eunotia veneris var. exsecta Clever-euler, that we found in our samples are new to Brazil. A total of 71 species was determined from 21 samples collected during the period of 36 months.