ABSTRACTSpeciation and species concepts were investigated for theCollybia dryophilacomplex. Mating compatibility studies show that all members of theC. dryophilacomplex have a heterothallic tetrapolar mating system and that several intercompatible groups are broadly distributed over several continents including North America, Europe and Asia. Intersterility between different groups is associated with isozyme differences. Isozyme variation between some pairs of intercompatible mating groups from different continents suggests that geographic isolation (allopatry) may be important during speciation in these fungi. Combined evidence from mating compatibility, isozymes, DNA hybridization, and morphology were used to assess patterns of divergence associated with speciation. Results of phylogenetic analysis indicate that speciation in these fungi is gradual and involves many steps that are not linked in any particular sequence with the development of prezygotic intersterility barriers. Varying patterns of speciation in these and other fungi have important implications for the species concept. Limitations of the biological species concept are discussed, and instead a phylogenetic based species concept is proposed, which provides information about evolutionary relationships among allopatric mating groups based on combined data from mating compatibility, morphology and macromolecules. New combinations are proposed forC. ociorssp.brunneolaandC. alpinassp.subsulphurea.