Halophilic nitrifiers belonging to the genus Nitrosomonas and Nitrospira were enriched from seawater and marine sediment samples of the North Sea. The maximal ammonia oxidation rate (AOR) in batch enrichments with seawater was 15.1 mg N L(-1) day(-1). An intermediate nitrite accumulation was observed. Two fixed-bed reactors for continuous nitrification with either polyethylene/claysinter lamellas (FBR A) or porous ceramic rings (FBR B)were run at two different ammonia concentrations, three different ammonia loading rates (ALRs), + or - pH adjustment,and at an increased upflow velocity. A better overall nitrification without nitrite accumulation was observed in FBR B. However, FBR A revealed a higher AOR and nitrite oxidation rate of 6 and 7 mg N L(-1) h(-1), compared to FBR B with 5 and 5.9 mg N L(-1) h(-1), respectively. AORs in the FBRs were at least ten times higher than in suspended enrichment cultures. Whereas a shift within the ammoniaoxidizing population in the genus Nitrosomonas at the subspecies level occurred in FBR B with synthetic seawater at an increasing ALR and a decreasing pH, the nitrite oxidizing Nitrospira population apparently did not change.