The toxic effects of triclosan (TCS) and nonylphenol (4-n-NP) on activated sludge heterotrophic and autotrophic microorganisms were evaluated. Toxicity experiments with specific oxygen uptake rate (SOUR) and ammonia uptake rate (AUR) revealed that TCS was much more toxic to heterotrophic and autotrophic microorganisms than 4-n-NP. In experiments with heterotrophic biomass, increase of sludge age (θ_c) from 5 to 15 days resulted in a decrease of median effective concentrations (EC₅₀) of TCS from 38.2 to 9.97mgl⁻¹ and in an increase of EC₅₀ values of 4-n-NP from 441 to 649mgl⁻¹. In experiments with autotrophic biomass and sludge age of 15 days, significantly lower EC₅₀ values were obtained for both compounds, indicating the higher sensitivity of nitrifiers to TCS and 4-n-NP. To compare toxicity of TCS and 4-n-NP towards single species and mixed wastewater cultures, experiments were performed using marine bacterium Vibrio fischeri. EC₅₀ values of 0.22 and 3.51mgl⁻¹ were estimated for TCS and 4-n-NP, respectively, indicating the higher sensitivity of this bioassay to toxicants. According to the levels of tested compounds commonly found in influent wastewater and the results of this study, there is a possible risk for deterioration of nitrification in activated sludge systems due to the presence of TCS.