Oxidation of high-strength ammonium wastewater can lead to exceptionally high nitrite concentrations; therefore, the effect of high nitrite concentration (>400 mM) was studied using an ammonium-oxidizing enrichment culture in a batch reactor. Ammonium was fed to the reactor in portions of 40–150 mM until ammonium oxidation rates decreased and finally stopped. Activity was restored by replacing half of the medium, while biomass was retained by a membrane. The ammonium-oxidizing population obtained was able to oxidize ammonium at nitrite concentrations of up to 500 mM. The maximum specific oxidation activity of the culture in batch test was about 0.040 mmol O 2 g −1 protein min −1 and the K s value was 1.5 mM ammonium. In these tests, half of the maximum oxidation activity was still present at a concentration of 600 mM nitrite and approximately 10% residual activity could still be measured at 1200 mM nitrite (pH 7.4), or as a free nitrous acid (FNA) concentration of 6.6 mg l −1. Additional experiments showed that the inhibition was caused by nitrite and not by the high sodium chloride concentration of the medium. The added ammonium was mainly converted into nitrite and no nitrite oxidation was observed. In addition, gaseous nitrogen compounds were detected and mass balance calculations revealed a nitrogen loss of approximately 20% using this system. Phylogenetic analyses of 16S rRNA and ammonium monooxygenase ( amoA) genes of the obtained enrichment culture showed that ammonium-oxidizing bacteria of the Nitrosomonas europaea/ Nitrosococcus mobilis cluster dominated the two clone libraries. Approximately 25% of the 16S rRNA clones showed a similarity of 92% to Deinococcus-like organisms. Specific fluorescence in situ hybridization (FISH) probes confirmed that these microbes comprised 10–20% of the microbial community in the enrichment. The Deinococcus-like organisms were located around the Nitrosomonas clusters, but their role in the community is currently unresolved.