Two acclimatized biomasses exposed to ammonium (NH 4 + ) concentration of 600mg NL −1 , one from a completely stirred tank reactor (CSTR), the other from a sequencing batch reactor (SBR), were assayed for nitritation performance, predominant nitrifying bacterial population and nitrous oxide (N 2 O) production. By virtue of fluctuating and constant NH 4 + concentrations respectively, the SBR and CSTR wastewater supply regimes were hypothesized to support different predominant ammonia-oxidizing bacteria (AOB) exhibiting distinct biokinetic properties. Nitritation efficiency (NO 2 − -N/NO 2+3 − -N) was higher in the SBR (89%) than the CSTR (30%) likely due to free ammonia and dissolved oxygen concentration. Quantitative fluorescence in situ hybridization (FISH) analyses revealed that fast-growing (r-strategist) AOB of halophilic and halotolerant Nitrosomonas lineage were more highly enriched in the SBR (76±4.2%) than the CSTR (38±6.0%). The CSTR predominantly enriched slow-growing (K-strategist) AOB Nitrosospira spp. (42±1.9% versus 1.4±0.8% in the SBR). Biokinetic parameter estimation consolidated the FISH result: the maximum growth rate and half-saturation coefficients for NH 4 + were higher in the SBR (μ max =0.92 day, K NH4+ =28.9mg NL −1 ) relative to the CSTR (μ max =0.42 day, K NH4+ =3.47mg NL −1 ), suggesting that the extent of nitritation may be controlled by choice of wastewater influent operational regime, which itself determines predominant AOB. N 2 O production was a maximum of 25 times higher (10.2mg N-N 2 Oh −1 at 0.5mg O 2 L −1 ) in CSTR-enriched biomass than in SBR-enriched biomass (0.41mg N-N 2 Oh −1 at 0.5mg O 2 L −1 ).