A lithotrophic ammonia-oxidizing bacterium of the Nitrosomonas type was isolated from the lower River Elbe. Enrichment was attained from suspended particulate matter (SPM) of a water sample. At its natural environment, this species almost exclusively occurred attached to flocs, as demonstrated with the immunofluorescence technique. On the species level, the isolate was not related to any of the described Nitrosomonas species. The strain was characterized by strong production of exopolymeric substances (EPS) and was observed to occur self-flocculating in pure cultures. Low ammonia concentrations stimulated EPS production. The EPS revealed an extensive capacity for binding particulate and dissolved materials, as well as cells of other bacterial species. This capacity was affected by changing pH values or salt concentrations of the medium. The EPS appeared to function as a buffer against toxic compounds and against changing environmental conditions. Another Nitrosomonas strain isolated from the Elbe estuary, but lacking recognizable EPS production, was used for comparison.