Enzymatic (x-glucosidase and peptidase activity in a nitrifying trickling filter (NTF) at the Rya wastewater treatment plant, Goteborg, Sweden, was investigated to evaluate whether these activities can be used as indicators of heterotrophic activity and polymer degradation. Samples of the biofilm were taken from the NTF and incubated in sterile filtered effluent water from the NTF with the addition of soluble starch, peptone, and ammonium chloride. In order to determine the distribution of enzyme activities, the alpha -glucosidase and peptidase activities were measured in the biofilm samples, in the filtered effluent water from the NTF and in the water phase in which the biofilm was incubated. Activities of both enzymes were found both in the effluent water from the NTF and in the biofilm. The enzyme activities were elevated in the samples when starch and peptone were present. In addition, there was a significant inhibition of ammonium oxidation in samples incubated with starch and peptone. Thus, the presence of starch, peptone and ammonium resulted in increased activity of heterotrophs, which lead to an inhibition of the nitrifiers, probably via competition for available oxygen.