The review deals with differences between European populations of Coregonus with respect to morphological and ecological traits, pre and postzygotic reproductive isolation, and characteristics of their parasitic fauna. Special attention is paid to the stability of the traits that are considered diagnostic for individual forms. It is concluded that the diagnosis of species of Coregonus on the basis of morphological and ecological traits alone may be problematic. Therefore, it is necessary to combine the results of morphological and ecological analysis with molecular genetic data.