This is a review of data that were obtained in the past and in more recent years by various authors who have often been involved in collective, interdisciplinary studies of the usefulness of phytoseston for evaluating the water quality of the Upper Vistula River. It is commonly known that drifting microflora in riverine bioseston is primarily a mixture of benthic forms that have been washed out from the bottom ? their prior habitat. A considerable contribution of allochthono-us forms were found among them. They originated from a reservoir created by river damming and also from tributaries. Generally, at the end of the 1990s at some Sites of the studied sector of the Vistula River there was intense development of potentially toxic cyanobacteria. The predominant taxa (e.g., Microcystis spp., Anabaena spp., Woronichinia naegeliana) and coccal green algae (e.g., Scenedesmus spp., Pediastrum spp.) are usually noted in eutrophic water bodies. These taxa competed with diatoms or sometimes prevailed over them in abundant populations. Since these groups were omitted from the benthic diatom index method, phytoseston analysis contributed very important additional information to the diatom index results applied by other authors.