Paracetamol is considered to be a non-steroid anti-inflammatory drugs (NSAID), which are amongst the most commonly used pharmaceutical mixtures worldwide. Accordingly so, it is found in almost all municipal waste waters in small but effective concentrations (ng/L up to μg/L). Paracetamol found in the environment degradates very quickly, however toxic effects on test organisms have been recorded. In this research we investigated the influence of paracetamol on the heterotrophic microfauna of activated sludge, under laboratory conditions over a period of 13 days. We used the following concentrations; 60, 120 and 180 mg/L of paracetamol. Paracetamol decreased the incidence and frequency of heterotrophic microfauna taxa in all treatments, especially the ones with concentrations of 120 and 180 mg/L. Ciliates Coleps hirtus, Drepanomonas revoluta, Trochilia minuta and the Rotifer Colurella uncinata proved to be exceptionally sensitive while Tetrahymena pyriformis and Vorticella microstoma proved to be least affected by the paracetamol. At the end of the experiment a recovery of the microfauna took place along with the degradation of paracetamol. Concentrations of 60 mg/L had temporary effects on the microfauna and after a period of 6 days, recovery was recorded. On the other hand, concentrations of 120 and 180 mg/L had irreversible changes on the composition of the microfauna. This research indicates the importance of using non-standard tests to assess the influence of xenobiotics, whereby examining should be concentrated on the level of biocenosis and should be held over longer periods of time.