Nitrified municipal wastewater effluent was passed upward through an anaerobic filter containing gravel media to effect denitrification. Using methanol as a supplementary carbon source for the denitrifying bacteria, 90% nitrogen removal was obtained with a detention time as short as 1.5 hr in pilot plant studies. Further laboratory studies were conducted using fiber media in upflow and horizontal flow filters. The highly porous fiber media performed comparably to the gravel media in the upflow filter; however, media configuration and poor flow distribution limited the success of the horizontal flow filters. The anaerobic filter process has several advantages over other methods of denitrification. These are low initial and operating cost, simplicity of operation, long solids retention times, and absence of any sludge recycle or disposal equipment. The major operating cost for the process is for the methanol. The quantity required increases with increasing dissolved oxygen and nitrate nitrogen and decreasing effluent BOD. The total cost, not including the cost of nitrifying the secondary effluent and reaerating the denitrified effluent, may run as low as $12.00 per million gallons.