In the environment, nutrients are rarely available in a constant supply. Therefore, microorganisms require strategies to compete for limiting nutrients. In freshwater systems, ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) compete with heterotrophic bacteria, photosynthetic microorganisms, and each other for ammonium, which AOA and AOB utilize as their sole source of energy and nitrogen. We investigated the competition between highly enriched cultures of AOA (AOA-AC1) and AOB (AOB-G5-7) for ammonium. Based on the gene, the newly enriched archaeal ammonia oxidizer in AOA-AC1 was closely related to spp., and the bacterial ammonia oxidizer in AOB-G5-7, sp. strain Is79, belonged to the Nitrosomonas oligotropha group ( cluster 6a). Growth experiments in batch cultures showed that AOB-G5-7 had higher growth rates than AOA-AC1 at higher ammonium concentrations. During chemostat competition experiments under ammonium-limiting conditions, AOA-AC1 dominated the cultures, while AOB-G5-7 decreased in abundance. In batch cultures, the outcome of the competition between AOA and AOB was determined by the initial ammonium concentrations. AOA-AC1 was the dominant ammonia oxidizer at an initial ammonium concentration of 50 μM, and AOB-G5-7 was dominant at 500 μM. These findings indicate that during direct competition, AOA-AC1 was able to use ammonium that was unavailable to AOB-G5-7, while AOB-G5-7 dominated at higher ammonium concentrations. The results are in strong accordance with environmental survey data suggesting that AOA are mainly responsible for ammonia oxidation under more oligotrophic conditions, whereas AOB dominate under eutrophic conditions. Nitrification is an important process in the global nitrogen cycle. The first step, ammonia oxidation to nitrite, can be carried out by ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB). In many natural environments, these ammonia oxidizers coexist. Therefore, it is important to understand the population dynamics in response to increasing ammonium concentrations. Here, we study the competition between AOA and AOB enriched from freshwater systems. The results demonstrate that AOA are more abundant in systems with low ammonium availabilities and that AOB are more abundant when the ammonium availability increases. These results will help to predict potential shifts in the community composition of ammonia oxidizers in the environment due to changes in ammonium availability.