Only three of nine test fungi formed ectomycorrhizae on Alnus glutinosa, A. incana, A. rhombifolia, and A. sinuata in pure culture syntheses: Alpova diplophloeus, Astraeus pteridis, and Paxillus involutus. Pisolithus tinctorius formed ectomycorrhizae to a very limited extent on Alnus glutinosa. It is suggested that the entire genus Alnus may be very specialized regarding its ectomycorrhizal fungus associates, yet, display little infrageneric mycorrhizal specificity.The role of phenolics and possible mechanisms determining ectomycorrhizal specificity are discussed, and the need for research on host–fungus recognition factors is emphasized.