Two morphologically and ecologically different forms within the genus Gammarus, known as G. plumicornis Costa, 1853 and G. aequicauda (Martynov, 1931), are interfertile in experiments and produce a vital F1. In nature, however, heterogeneous couples are rare, although not altogether absent. It is supposed that plumicornis and aequicauda are two former subspecies, which – owing to an overlap in their present distribution areas – have not developed sufficient isolating mechanisms to prevent hybridization.