The liquid phase of different units of an underground gas storage (UGS) in the period of gas injection was studied with respect to its hydrochemical composition and characterized microbiologically. The presence of viable aerobic and anaerobic bacteria was revealed in the UGS stratal and associated waters. An important source of microorganisms and biogenic elements in the ecosystem studied is water and various technogenic admixtures contained in trace amounts in the gas entering from the gas main in the period of gas injection into the storage. Owing to this fact, the bacterial functional diversity, number, and activity are maximal in the system of gas treatment and purification and considerably lower in the observation well zone. At the terminal stages, the anaerobic transformation of organic matter in the UGS aqueous media occurs via sulfate reduction and methanogenesis; exceptionally high rates of these processes (up to 4.9 x 10(5) ng S(2-)l(-1) day(-1) and 2.8 x 10(6) nl CH4 l(-1) day(-1), respectively) were recorded for above-ground technological equipment.