This paper discusses the influence of a wide range of anionic surfactant concentrations on activated sludge. Linear alkylbenzene sulphonate (LAS) was chosen as an example of a commonly used anionic surfactant. The fate of the surfactant during biological treatment of wastewater was tested. The effect of surfactant on glucose and starch removal was studied. It has been found that in the case of glucose the removal was independent of LAS concentration, while in the case of starch it was incomplete at high surfactant loads (above 15 mg·(g·dss)^-1). The study established that surfactants can activate or inhibit microorganism activity, depending on surfactant concentration. LAS loads up to 3 mg·(g·dss)^-1 positively stimulate the removal of COD, phosphorus release and the respirometric activity of the sludge. LAS loads higher than 15 mg·(g·dss)^-1 inhibit respiration of activated sludge bacteria and decrease phosphorus removal. It also affects the morphology of activated sludge flocs, causing their fragmentation and lysis of protozoa cells.