A method is presented to derive an operational phenetic species concept for the Hebeloma crustuliniforme complex in northwestern Europe. The complex was found to consist of at least 22 biological species (intercompatibility groups; ICGs). Almost none of these biological species could be recognised unambiguously by morphological criteria. It is therefore necessary to base a phenetic species concept on combinations of biological species. However, such species delimitation must be performed within an explicitly phylogenetic context. It is crucial therefore to have a reliable estimate of the phylogeny of 22 biological species in that complex. Based on two nuclear sequences, we present a best estimate of the phylogeny of biological species within the complex. Using this phylogeny, on the basis of strict monophyly only two species can be morphologically recognised among 22 biological species. Relaxing the criterion of monophyly and allowing paraphyletic groupings of biological species as phenetic species would result in the recognition of three phenetic species. A tree, with the five ICGs of the previously defined morphospecies H. crustuliniforme (1, 2, 3, 4 and 5) constrained as a monophyletic group, can not be rejected. This constrained tree, together with the relaxed criterion that allows for paraphyletic groupings of biological species, leads to the recognition of four phenetic species, viz. H. crustuliniforme, H. helodes, H. incarnatulum and H. velutipes. These phenetic species are described and a key is provided. Other taxon names are briefly discussed. The very limited ability to translate a biological species concept into an operational phenetic species concept is explained by the lack of qualitative characters and the plasticity of quantitative characters. Recency of common evolutionary history is also a major factor. Intercompatibility tests and DNA based phylogenies indicate that most biological species are very closely related and hence provide support for the claim that correspondence between a biological species concept and a phenetic species concept in the H. crustuliniforme complex is not likely to be forthcoming. In an Appendix morphological descriptions are provided of the 22 ICGs.