Activated sludge is a biomass of organisms used in biological treatment of wastewater. Various industrial plant wastewaters can be laden with salts and seawater intrusions into wastewater treatment plants is also a possibility. So an experiment was carried out in order to study the effects of sodium chloride (NaCl) on communities of organisms in activated sludge. Different quantities of NaCl were added to samples in order to attain concentrations of 5, 7 and 10 ‰. Experiment lasted for two weeks and the activated sludge used was taken from the Central Wastewater Treatment Plant for the city of Zagreb. In all treatments, sodium chloride had an effect on both composition and structure of the communities. With increasing concentrations of NaCl, a decrease of the number and density of species in activated sludge microfauna was observed. This implies possible unfavorable effects on the efficiency of activated sludge. Species that proved resistant to sodium chloride were the holo-euryhaline species of ciliates, namely Cyclidium glaucoma and Acineria incurvata. These species were also dominant in samples containing NaCl. Most sensitive species proved to be the oligo-stenohaline species of ciliates (Coleps hirtus, Aspidisca cicada, Drepanomonas revoluta) as well as all the species belonging to the groups Suctoria and Rotifera.