Genus Nitrospira and Nitrobacter species are the key nitrite-oxidizing bacteria (NOB) in nitrifying wastewater treatment plants. It has been hypothesized that genus Nitrospira are K-strategists and can exploit low amounts of nitrite more efficiently than Nitrobacter. In contrast, Nitrobacter species are r-strategists that can grow faster than Nitrospira. To elucidate the K/r hypothesis and to analyze the effect of substrate (nitrite) concentration on the competition and distribution of the two NOB, two different reactor types were employed for nitrite oxidation (nitratation) and NOB growth. The continuous biofilm airlift reactor (CBAR) maintained low nitrite concentration due to the complete oxidation of nitrite in continuous operation while the sequencing batch reactor (SBR) was kept in a relatively high nitrite concentration environment due to a cyclic substrate concentration profile. Fluorescence in situ hybridization (FISH) and confocal laser scanning microscopy (CLSM) showed that both Nitrobacter species and genus Nitrospira were present in the CBAR and the SBR. Quantitative FISH analyses of the CBAR showed that Nitrospira occupied 59% of the total bacteria while Nitrobacter occupied only 5%. On the other hand, Nitrobacter, occupying 64%, was the dominant NOB in the SBR, and only 3% of total bacteria belonged to genus Nitrospira. Nitrite oxidation kinetics and quantitative FISH analyses revealed that the specific nitrite oxidation activities of Nitrobacter and Nitrospira are 93.8 and 10.5 mg/g NOB h, respectively, and the specific activity of Nitrobacter is about 9 times higher than that of Nitrospira. In conclusion, the results confirm the K/r hypothesis and the distribution of Nitrobacter and Nitrospira is likely to depend mainly on nitrite concentration. It seems that nitrite load and starvation conditions do not give a direct effect on the distribution of NOB.