Abstract: Underground and surface water are contaminated by nitrate in several ways. Nitrate originated from agriculture is increasingly growing all over the world due to the extreme use of fertilizers. Nitrate salts reach the groundwater as they percolate through the soil. Some other sources of nitrate in ground and surface water are from uncontrolled discharges of treated or un-treated wastewater from domestic and industrial sources, landfills and animal waste particularly from animal farms. Various methods for treatment of water from nitrates are known, but the majority of them generate secondary pollutants. An exception is the biological denitrification, at which nitrates are reduced to harmless nitrogen gas, and side waste products practically do not occur. In this paper the influence of nitrite ions on the process of denitrification was investigated. It is established that the denitrification process depends on the ratio between nitrate and nitrite concentration. The process is inhibited even at low nitrite concentration.