The numbers of hydrocarbon-oxidizing and ammonifying microorganisms (psychrophilic and mesophilic) in the soils of the aeration zone and groundwater polluted with petroleum products under application of mineral fertilizers were analyzed. In the soils of the aeration zone, the numbers of aerobic microorganisms increased by 1-2 orders of magnitude, ammonifying ones – up to 107 CFU/g, hydrocarbon oxidizing microorganisms – up to 106 CFU/g, at the same time numbers of psychrophilic microorganisms increased by 13%. In groundwater the numbers of psychrophilic hydrocarbon-oxidizing microorganisms increased from 104 to 107 CFU/g, mesophilic ones from 105 to 107 CFU/g; the numbers of ammonificating microorganisms increased from 104 to 108 CFU/g and from 105 to 107 CFU/g, respectively. The number and variety of protozoa also increased. In the soils only flagellates of Bodo genus were found, their numbers during processing changed from 10-102 cells/g to 103 cells/g; in groundwater from 102–103 cells/ml to 104 cells/ml. The number of ciliata (Ciliata, Uronema genus) in groundwater changed from tens of cells per ml to 102–103 cells/ml. After treatment, ciliata were detected in all water samples, and in addition to Uronema genus ciliata of Colpoda genus were discovered. The biostimulation produced changes in the chemical composition of groundwater (ammonium concentration, permanganate oxidizability of water), that indicated the decomposition of petroleum products and the appearance of easily oxidizable organic substances in water. The concentration of petroleum products in groundwater over the course of 3 years has decreased from 500–120 mg/dm3 to 10–1.5 mg/dm3.