Microorganisms are normal inhabitants of natural water sand have a distinct role in the aquatic environment. Some of them have a vitally important role in the aquatic ecosystem, other important and interesting because of the grow thin wastewater and the refore represent a potential threat to water supplies. Microbial populations populationsie. Their presence in the aquatic environment depends on the physical and chemical factors including light, hydrostaticpressure, pH, salt concentration and temperature. Water contaminated waste water contains a relatively large number of microorganisms. Some of the microorganisms present in the water can be pathogenic. Coliformbacteria include Escherichia coli, Enterobacteraerogenes, serve as indicators for faecal contamination. For the determination of coliform bacteria in the water are two usual procedures; determine the most probable number of Most Probable Number (MPN or) and membrane filtration. Besides coliform bacteria as in dicators of faecal contamination can be used Clostridium perfringens, Streptococcus faecalis. The water that is contaminated apply certain procedures. Vodovod-Osijek Ltd. Carried out medical control of drinking water in the internal laboratory of the company, which owns the authority of the Ministry of Health. From microbiological analyzes are performed as follows: total bacteria, total and fecal coliforms, faecal streptococci, bacterias of species Pseudomonas aeruginosa, Escherichia coli, bacterias from rod of Salmonella and other enterobacteria.