p-Me(CH(2))(11)C(6)H(4)SO(3)Na was added (up to 1000 mg/L) to an industrial activated sludge under aeration in a lab. bioreactor (simulating a municipal sewage treatment plant) together with an artificial culture medium to study the effect of the surfactant on the microorganisms (bacteria, protozoa) occurred in the sludge. The addn. of the surfactant to the concn. higher than 50 mg/L resulted in a limitation of the microorganism growth (or even in a full atrophy of the microorganisms at the conc. 1 g/L) but the sewage life was reconstituted after the addn. was stopped. Quite high degree of the surfactant decomp. (40%) was achieved in every case.