Diversity of nitrifying bacterial population was investigated in sludge samples taken from a full-scale biological wastewater treatment plant (WWTP) treating domestic wastewater by fluorescent in situ hybridization (FISH) during seasonal operation. Duplicate grab samples were collected in March 2003, June 2003, December 2003 and May 2004 from the aerobic tank of the treatment plant. FISH results were interpreted with system performance in terms of BOD5, TKN and NO3-N removals and also with operational parameters such as wastewater temperature and sludge age. BOD5 removal efficiencies were always greater than 90% whilst TKN removal in a range of 69-95% were achieved during the monitoring period. Although there were variations in operational conditions of the biological treatment system both Nitrosomonas and Nitrosospira genera from AOB and Nitrobacter genus from NOB were found to be present in all samples examined.