The sympatric occurrence of two bathynellids previously considered races of Bathynella natans - natans and stammeri - is evaluated as a natural ecological-genetic experiment. Since no hybrids appear in mixed populations, these forms are proven to be full species: Bathynella natans Vejdovsky and Bathynella stammeri (Jakobi). Besides the form of the mandibles, which until now was the only taxonomically useful diagnostic character in the genus Bathynella, 7 additional, suitably applicable morphological characters have been found (Table 3). The Bathynella biotope investigated is assigned to the “eustygopsammal” subterranean life province (Husmann 1966), which is associated with the “Parastenocaris-Bathynella” biocoenosis (Husmann 1962). This particular biocoenosis is evidently resistant to organic pollution of ground water. The sympatric existence of Bathynella natans and B.stammeri can be explained by consideration of the geo-limnological developmental history of the interstitial zone of the North German low plain. Sands and gravels were widely deposited in the North German Basin by northward-retreating glaciers, creating microcavernous living space and passages for the interstitial fauna. This microfauna could find passages in layers of sand under and along the northward-flowing streams. Primitive Ice-Age streams (,,Urstromtäler” of Keilhack) formed east-to-west cross-connections between the south-north distributional corridors. The great geographical expansion of the tributary river courses which reached the north German plain before, during, and after the Ice Age suggests that ground water habitats were temporarily separated and later rejoined by orogenic movements of the earth’s surface. Such an orogenically caused, geomorphological isolation lasting for a sufficiently long geological period could have led to the result that species, originating in isolation from the same phylogenetic stock, subsequently were brought together again in the same biotope. This is particularly true for bathynellids, which as archaic types (Lebensformtypen) of the ancient, extreme “mesopsammal” biotope (Remane) are quite likely to have become sympatric in such a manner.