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The presently known ecological valences and requirements of recent species of pill-clams (Pisidium C. Pfeiffer) in Central Europe are briefly reviewed. Species of low indicator value (e. g. P. subtruncatum, milium) are contrasted with those having a high indicator value (e. g. P. supinum, lilljeborgii). The necessity of taking into account the abundance of species, rather than merely their presence, is emphasized.