The influence of predators on nitrification in aerobic biofilm processes was investigated in a laboratory study carried out using two aerobic continuous-flow suspended-carrier biofilm reactors operating in parallel on the same synthetic waste water. After nitrification was established in the reactors and stable operating conditions at 3 h hydraulic retention time were achieved, nystatin and cycloheximide, substances inhibitory to eucaryotic organisms, were added to one of the reactors to selectively inhibit the predators. The other reactor was operated as a reference, without inhibitors being added. Adding the inhibitors to the test reactor led to a rapid decrease in the quantity of biofilm-consuming predators, most of them rotifers and nematodes, and a simultaneous increase in nitrification, which finally stabilized at a level twice as high as in the reference reactor. The addition of inhibitors was then switched between the reactors, resulting in a rapid increase in nitrification in what had been the reference reactor, and a slow decrease in nitrification in the reactor in which the adding of inhibitors had been stopped. Again, approximately twice as high a nitrification rate was achieved with addition of inhibitors than without. The results clearly show that biofilm predators can have a strong negative effect on nitrification in aerobic biofilm processes.