Results of field observations, cross-breeding experiments, morphometry, and starch gel electrophoresis suggest that A. cinereus and A. najas are specifically distinct. Cross-breeding experiments between the two taxa in the laboratory were only successful when using males of A. cinereus and females of A. najas. Field observations on mixed populations in spring did not reveal any pre- or postcopulae involving partners of the two taxa (assigned by body length). Instead all pairs observed consisted of conspecific males and females. Morphometry of field caught adults of A. najas and A. cinereus yielded a clearly bimodal distribution of body length with only very few individuals of intermediate size. Therefore, body length alone allows reasonable separation of the two taxa in natural populations. Although F1-hybrids from laboratory rearings (photoperiod 18L:6D, temperature ∼ 23°C) differed only marginally in body length from A. najas reared under the same conditions, backcrosses and also F2-hybrids were of intermediate size. In the field individuals of both taxa are predominantly wingless, but both hybrids and backcrosses from the laboratory showed a net shift in morph ratio with many longwinged specimens. Vertical starch gel electrophoresis of three monomorphic (Apk, Got-2, Mdh-2), six polymorphic loci with low levels of polymorphism (Idh-1, Fum, Got-1, Mdh-1, Me, 6-Pgd) and three highly polymorphic loci (Es-4, Idh-2, Pgm) suggests that the two taxa do not share a common gene pool. Gene differentiation among all A. cinereus populations as measured by Nei's GST values is very low. Between Tyrrhenian and continental A. najas populations, however, gene differentiation is very high, and in the same order of magnitude as gene differentiation between the two taxa A. cinereus and A. najas in sympatric populations. Our field observations, together with morphometric data, laboratory rearings, and information on wing polymorphism, suggest that hybridization in natural populations is rare.