Nitrifiers are known to form relatively dense and strong microcolonies in activated sludge flocs, but little is known about their adhesion characteristics and how these are relative to other floc components. The size distribution of ammonia and nitrite-oxidizing bacteria (Nitrosomonas oligotropha and Nitrospira spp.) in activated sludge from a nutrient removal plant showed that the majority of N. oligotropha cells formed microcolonies with a diameter of 13-22.5 microm, and most Nitrospira spp. cells formed microcolonies with a diameter of 9-22.5 microm. By applying high shear forces (2200 s(-1)), the largest microcolonies of N. oligotropha fragmented to a level well below the Kolmogorov microscale (approx. 15-25 microm). Only very little erosion of single cells took place. Nitrospira spp. microcolonies were generally slightly stronger than N. oligotropha. In order to characterize the adhesion/binding mechanisms for the individual microcolonies, a number of different physico-chemical treatments were combined with shear, and even though this did not lead to any explicit characterization of the species-specific adhesion mechanisms, entanglement of extracellular polymers was proposed as a plausible important adhesion mechanism. When compared to other floc components, the deflocculated fractions of N. oligotropha and Nitrospira spp. were much lower than those of cells in general (total cell count, DAPI) or the organic matter. Deflocculation of N. oligotropha ranged from 3% to 11% of the total N. oligotropha biovolume, Nitrospira spp. from 1% to 4% of the total Nitrospira spp. biovolume, whereas the number of deflocculated cells was 9-54% of the total cell count, and the deflocculated organic matter constituted 8-43% of the total amount of organic matter. These results clearly showed that activated sludge contained a large pool of loosely attached cells and extracellular polymeric substances, and that the nitrifiers and some other microcolony formers were very strong and remained almost intact even under extreme physical and chemical conditions.