The U.S. Public Health Service Drinking Water Standards of 1962 recommend that nitrates in drinking water be limited to 45 milligrams per liter. This limit has been set because of the association of high nitrates in water with the incidents of methemoglobinemia in infants. The results of investigations on the removal of nitrates from irrigation return waters in the San Joaquin Valley in California have indicated that biological denitrification may be an economically feasible way to remove nitrates. The purpose has been to develop methods to control the discharge of undesirable quantities of nitrates into the San Francisco Bay System rather than to develop a process for nitrate removal from a municipal water supply. However, of the several processes studied, biological denitrification in filter beds has proved to have the stability and efficiency of operation that would be necessary for treatment of a drinking water supply. The paper contains a summary of the pilot-plant studies which were conducted to estimate chemical requirements, to evaluate possible filter media, and to determine the important operating parameters.