A new species, Rhithrogena grischuna nov. sp., is described at all stages (imagines, subimagines, nymphs and eggs) from material collected in eastern Switzerland. This species belongs to the hybrida group, and is related to Rh. hercynia and allied species, for which we propose a new group, the hercynia group, characterized by the presence of dark spots on the upper face of the femora, both in imaginal and larval stages. Some ecological and biocoenotic data on Rh. grischuna are also given.