The Desna is one of only a few rivers in Ukraine with an undisturbed hydrology and surrounding landscape. Along 1162 kilometers of its length only three reservoirs have been built, and they are relatively small. During more then ten years we studied the phytoplankton and phytobenthos of the Desna river and some of its tributories for their species composition. In total, 388 algal species from 7 divisions were identified. The most common were species of the genera Amphora, Cocconeis and Navicula. In the Desna, most algal communities are polydominant. Only in summer in shallow water does one encountere communities of blue-green algae with dominance of Oscillatoria limosa Ag. and Oscillatoria amphibia Ag. In some cases abundant growth of Hydrodiction reticulatum Roth. was observed. The discovery of the red alga Thorea sp. should be noted. Solitary individuals of this alga occurred rarely in the lower part of the Desna: this is a new species for the left-bank Polissia. There are many flood-land waters and bogs in the basin of the Desna. Most are located on reserves. Their ecosystems thus remain in a natural state, scarcely changed by human influence, and they are reservoirs of biodiversity in the Desna basin.