In a biological contactor that is part of the biological pretreatment of landfill leachate in Mechemich (Germany) nitrogen elimination of 60% or more was observed under low dissolved oxygen (DO) conditions. Ammonia was converted without accumulation of nitrite and with only little nitrate production. Interestingly, due to limited supply with organic substrate m the system, this observation cannot simply be explained by a combination of conventional autotrophic nitrification and heterotrophic denitrification. In situ hybridization with 16S rRNA-targeted probes revealed the presence of large microcolonies of at least three different types of ammonia-oxidizing bacteria m those biofilm regions where extremely high nitrogen losses occurred. These results were confirmed by comparative sequence analysis of biofilm-derived amoA (encoding the active-site polypeptide of ammonia-monooxygenase) clones for molecular fine-scale analysis of the ammoma-oxidizing population. In batch tests inoculated with biofilm material nitrogen loss occurred without dosage of organic substrate at a DO concentration of 1 mg/l. The simultaneous presence of ammoma and nitrite m the reactor induced the process of complete nitrogen elimination. N2 was identified to be the gaseous end product of the reaction. These results indicate that under low DO concentrations autotrophic ammonia-oxidizers might be the causative agents of the observed nitrogen loss by performing aerobic/anoxic denitriftcation with nitrite as electron acceptor and ammonia (or perhaps hydroxylamin) as electron donor.