Aims: To obtain ammonia-oxidizing bacterial (AOB) strains inhabiting low dissolved oxygen (DO) environments and to characterize them to better understand their function and ecology. Methods and Results: Using a serial dilution method, two AOB strains (ML1 and NL7) were isolated from chemostat reactors operated with low DO concentrations (0.12-0.24 mg l(-1)). Phylogenetically, strains ML1 and NL7 are affiliated to AOB within the Nitrosomonas europaea and Nitrosomonas oligotropha lineages, respectively. Kinetically, strain ML1 had high affinity for oxygen (0.24 +/- 0.13 mg l(-1)) and low affinity for ammonia (1.62 +/- 0.97 mg N l(-1)), while strain NL7 had high affinity for ammonia (0.48 +/- 0.35 mg l(-1)), but a surprisingly low affinity for oxygen (1.22 +/- 0.43 mg l(-1)). A co-culture experiment was used to iteratively estimate decay constants for both strains. Conclusions: The results indicated that AOB without high affinity for oxygen may have other mechanisms to persist in low DO environments, with high affinity for ammonia being important. Significance and Impact of the Study: This study provides a method to determine AOB growth kinetic parameters without assuming or neglecting decay constant. And, this is the first report on oxygen affinity constant of a N. oligotropha strain.