Cultures of sulphate-reducing bacteria from subterranean waters were divided into three groups, on the basis of their sensitivity to the sodium chloride concentration. The optimal sodium chloride concentration for group D1 cultures was 1% and under and the maximum tolerated concentration was 3.7%; in group D 4 cultures the optimal and maximum NaCl concentration were about 2% and 5.6% respectively and in group D 7 cultures about 3.5% and over 7% respectively. On using nutrient media containing 4% and 7% NaCl, these groups could be separated from a mixture and the bacterial count for each individual group could be determined. Study of the adaptation of these cultures to high sodium chloride concentrations showed that the group characteristics of the culture remained constant for at least ten passages. Comparative study of typical natural waters showed an incidence of D 1 bacteria in fresh water, but not in brackish water and sea water. The two latter types contained group D 4 and D 7 bacteria, the proportion of the latter group increasing with the degree of mineralization of the water. Study of the incidence of groups of sulphate-reducing bacteria was combined with other microbiological and hydrochemical indicators for resolving questions of the genesis of subterranean waters in Carpathian flysch. The results showed that the presence of variously mineralized hydrogen sulphide waters of different origin as regards their connection with the surface could be determined on the basis of the given criteria. The resultant picture corresponds to the recent state of the water and thus does not permit determination of the sedimentation system of the given water-bearing horizons.