This work comprises the results of investigations on the amount of heterotrophic bacteria (mesophilic), bacteria showing the sanitary state (TC, FC, FS), anaerobic spore-forming bacteria (Clostridium perfringens), pathogenic/ potentially pathogenic bacteria (Staphylococcus aureus, Pseudomonas aeruginosa, Salmonella spp.), filamentous fungi and yeasts and parasitic worms in sewage sludge at sewage treatment plants in Olsztyn, Olsztynek and Kętrzyn in the Mazurian Lake District. The number of FC, Clostridium perfringens and Salmonella as well as parasitic worms in the soils fertilized by 6.7 and 13.4 t/ha dry wt. of sewage sludge from deposit fields at the sewage treatment plant in Olsztyn was also determined. The lowest degree of bacteriological contamination was found in sewage sludge from the sediment fields at the sewage treatment plant in Olsztyn, whereas the highest degree was found in sewage sludge from lagoons at the sewage treatment plant in Kętrzyn. The soils fertilized by sewage sludges from sediment fields at the sewage-treatment plant in Olsztyn contained 100 - 100,000 times fewer FC, Clostridium perfringens, and parasitic worms than sewage sludge. There was a lack of pathogenic bacteria from the group of Salmonella. The numbers of FC, Clostridium perfringens, and parasitic worms in soils depended on the amount of sewage used in fertilization. This suggests preliminary hygienization of sewage sludges before they are used in agriculture.