The new subspecies Stenasellus skopljensis thermalis, from Banja Luka (Bosnie, Yugoslavia) is described. From the ecological point of view this form differs from the others because it inhabits underground waters of elevated temperature (240°C). Another constantly abundant species, St. hungaricus thermalis Mestrov, also occurs in Yugoslavia under the same ecological conditions, in the warm springs of Podsused near Zagreb. This indicates that these underground waters at elevated temperature are not accidental but preferred habitats for these forms, and confirms once again that thermal waters of this type are the biotopes-refuges in which certain relic forms are retained.