Landfill leachates were treated using a two-stage rotating biological contactor (RBC). At an ammonium load of 1.92 g N-NH(4)/m(2) d, complete nitrification was obtained at the first stage; however, at a higher load (3.6 g N-NH(4)/m(2) d) a two-stage system was needed to obtain complete nitrification. A further increase in the ammonium load to 4.79 g N-NH(4)/m(2) d and 6.63 g N-NH(4)/m(2) d caused a decrease of overall nitrification efficiency to 74.4% and 71.6%, respectively. Randomly amplified DNA analysis of bacteria present in the RBC revealed that the bacterial community differed over time. Regardless of the ammonium load, Nitrosomonas europaea and Nitrosomonas eutropha were the dominating species. The performed analyses provide a clear picture of bacterial population changes in response to changes of ammonium load during landfill leachate treatment. (C) 2009 Elsevier Ltd. All rights reserved.