Thanks to intensive exploration and to new methods for capturing aquatic underground fauna, 117 localities are now known for Stenasellus virei. The description of some typical biotopes suggests that the species lives as well in karstic waters as in phreatic ones, inside the different environment of the hydrogeological classification of subterranean waters. St. virei buchneri and St. v. hussoni are almost cavernicolous. St. v. angelieri is distributed in the underground waters of Catalonia. St. v. boui is located in the underflow of Salat river basin. St. v. virei is widely distributed in the alluvial water-level of Garonne and Ebro rivers basins. The dispersion of St. virei into the alluvial environment explains the process of colonization of continental underground waters. It explains also the existence of an apparently insulated population into the sink-hole of Padirac. The actual distribution of the five subspecies is explained by important restrictions of the area in quaternary glacial ages, followed by local (in the water-level of the tributaries of Garonne river) spreading during postglacial time. The postglacial reconquest of the Salat river underflow by this species seems to have been responsible for the latest subspeciation (St. v. boui). The endemic populations of fossil karstic systems seem to have an abnormal composition. They include unusually large adults, juvenile stages being rare. They differ from the phreatic populations, which exhibit a normal distribution is size groups, with a formal percentage of juveniles. These differences between karstic and interstitial populations may result from the fact that in caves, St. virei is often insulated from its original phreatic biocoenosis: an intraspecific competition between size classes has taken the place of normal heterospecific struggle for existence.