Most of our knowledge about the physiology of ammonia-oxidizing bacteria is based on experiments with Nitrosomonas europaea, which appears to be less ubiquitous than Nitrosospira. We have isolated Nitrosospiras from widely different environments and compared their specific growth rate, substrate affinity, urease activity, temperature response, pH tolerance and cell morphology. Two of the strains had a variable morphology: the spirals were less tightly coiled than the classical Nitrosospira type and a fraction of the culture had a vibrioid appearance. These vibrioid strains were also peculiar in having a much higher apparent activation energy for ammonia monooxygenase (AMO) (129 and 151 kJ mol(-1)) than that of the more classical Nitrosospiras (78 and 79 kJ mol(-1)). The differences in morphology and activation energy were congruent with the phylogeny of the genes for 16S rRNA (Utåker et al., System. Appl. Microbiol. 18) and AMO. The response to pH in the medium was investigated for four strains. The oxidation rate at the onset of the pH exposure experiment was found to obey classical steady state enzyme kinetics, assuming that NH(3) (not NH(4)(+)) is the rate-limiting substrate. The calculated half saturation constants (K(s)) for AMO were 6-11 µM NH(3). Growth had a narrower pH range than oxidation activity and appeared to be restricted by pH-dependent factors other than NH(3). All the isolated strains were urease positive, with a specific urease activity ranging from 60 to 158% of their specific AMO activity. The urease activity was unaffected by acetylene inhibition of the energy metabolism. The substrate affinity for one strain was found to be around 670 µM.