The effect of salt on the nitrification activity in full-scale wastewater treatment plants (WWTP) was investigated. Not only the activity of ammonia and nitrite oxidisers was measured, but also the nitrifying population was assessed (by Fluorescent In Situ Hybridisation)--in full-scale domestic and industrial WWTPs, operated under various salt levels. The results demonstrate a decline in the activity of ammonia and nitrite oxidisers with an increase in salt content: the domestic WWTP with the lowest salt level (0.13g Cl- l(-1)) had the highest specific activity of ammonia and nitrite oxidisers (4.3 and 2.4 mg N (gVSS)(-1) h(-1), respectively), while the lowest specific activities of ammonia and nitrite oxidisers (1.1 and 0.5 mg N (gVSS)(-1) h(-1)) were measured at the highest NaCl concentration (16g Cl- l(-1)). However, comparing the nitrification activity of different types of sludge developed under different operational conditions with the reported values was not directly possible. So we have used the activated sludge model (ASM) to translate the routine operational data into parameters to enable the calculation of the actual fraction of nitrifiers and consequently the actual specific activity of ammonia and nitrite oxidisers. Expressing the activity of ammonia oxidisers in terms of actual specific activity makes the results from pure cultures, enriched cultures, pilot scale and full scale WWTPs comparable. Moreover, these results confirm the behaviour of nitrifiers under salt stress and validate the results obtained from pure and enriched cultures to be extrapolated to full scale.