Many cavernicolous and phreatic localities are known for the species Stenasellus virei. Some of these, which harbor a rather abundant population have been studied for several years. The endemic populations from permanent waters of some fossil karstic systems seem to have an abnormal composition. They include especially large individuals (juvenile stages being rare). They differ from the phreatic populations, which exhibit a normal distribution in size groups with a normal percentage of juveniles. These differences in the structure of populations may result from physical differences between the habitat in free waters of caves and in phreatic water, and from differences between the associations of species that these two types of hypogean habitat may support.