The concentration of anionic surface active substances (ASAS) in the water of Lake Jeziorak Mały and the degradation of these substances by bacteria isolated from the surface microlayer and subsurface water were the object of the research of this paper. The concentrations of ASAS in water of this lake were at the level of about 2 mg/1. The biodegradation of ASAS was performed twice as well by microorganisms from the surface microlayer than from subsurface water. Moreover, within the surface microlayer the process of decomposing ASAS was most effectively performed by bacteria isolated from the layers of water closest to the surface (100-150 µm).