Two corn. p-HO(CH(2)CH(2)O)(x)C(6)H(4)CMe(2)CH(2)CMe(3) (x=9-10) (I) and p-Me(CH(2))(11)C(6)H(4)SO(2))OHa (II) surfactants were added up to 1000 mg/L to activated sludge from a municipal sewage treatment plant and studied for their biodegradn. in a flow system (modified Husmann app.). Sludge concn., sedimentation rate, sludge index and surfactant concn. were detd. The removal of the anionic I surfactant after 40 days of the test was less efficient than the removal of nonionic II surfactant.