AbstractInternational and national protection strategies and directives focus mainly on macroscopic organism and attempt to maintain their endangered habitats. However, microscopic communities are also threatened by decreasing biodiversity and many species including freshwater algae can disappear without even knowing they were present in the habitat. Defining rarity of microscopic taxa is not easy. The species' rarity is based on detailed knowledge of distribution and abundance of species. But

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... nly limited information is available about rare algal species especially in a given ecoregion. Reducing the data gaps, here, we present altogether 20 phytoplankton taxa rare in Hungary: three species of Chlorophyceae, eight species of Trebouxiophyceae, two taxa of Euglenophyceae, one-one species of Cyanobacteria, Bacillariophyceae and Mediophyceae and three species of Xanthophyceae. One of them, the Cylindrotheca gracilis is on the Hungarian Red List. Physical and ecological characteristics of standing waters where these species were found as well as their former occurrence all over the world are also reviewed.