Laboratory studies were conducted on a sequencing batch reactor (SBR) for the treatment of screened liquid pig manure. The SBR was operated on the basis of a 24 h cycle at 6 and 90 days hydraulic retention time and 20 days solids biological retention time. Nitrification and denitrification processes were carried out in the same reactor by alternating aerobic, anaerobic conditions, and the tested time ratios were 19/3, 16/6, 14/8 and 10/12 (hours). The results from the overall performance indicated that above 99% NH3-N, 93% total Kjeldahl nitrogen (TKN), 97% chemical oxygen demand (COD) and 97% total suspended solids (TSS) removal were achieved in the SBR treatment. Anaerobic operation did not adversely affect the activity of the nitrifiers, and NO2-NO3-N concn increased up to 230 mg/litre following air supply. However, sludge settling was poor in those reactors functioning with an anaerobic period greater than 8 h. As a result of nitrification and denitrification the removal of ammonia and nitrite plus nitrates nitrogen was 80 to 93%.