The rates of denitrification and biological nitrogen fixation of the periphyton communities associated with the culms and adventitious root of Oryza glumaepatula in Lake Batata, State of Pará, Brazilian Amazonia, were investigated over the course of 24 h. N2O was not detected during the period, demonstrating that denitrification was not occurring. The low concentration of nitrate in the water column was indicated as the main factor limiting the denitrification process. The rates of biological nitrogen fixation (BNF) in culms were lowest at night and early morning, while the highest values were measured at 1200 h. BNF rates in periphyton associated with adventitious roots were lowest during the morning, while highest values were measured at 1600 h. The difference in the daily patterns of BNF rates in the periphyton of culms and roots of O. glumaepatula in the euphotic zone was attributed to defferences in the availability of organic carbon which supports this process. In the periphyton of the culms, the BNF process was carried out by heterocystous cyanobacteria, which was photoautotrophs. In the periphyton of the adventitious roots, BNF was carried out by periphytic bacteria, and is heterotrophically mediated.