On the siliceous erratic boulder "Alexanderstein" in Küsnacht (Canton of Zürich, Switzerland) 48 bryophyte taxa were identified. Their ecological indicator values and other ecological traits were analysed. The mosses Dicranum fulvum, Hedwigia ciliata, Grimmia decipiens, G. elatior and G. hartmanii as well as the fern species Asplenium septentrionale are remarkable, because on the Swiss Plateau and in the Jura region these calcifuge rock-dwelling species exclusively inhabit siliceous erratic boulders. The populations of these species are, however, very small and they are potentially threatened by recreational use of the boulder. Conservation measures are discussed.