The aim of this study was to determine the impact of a static magnetic field (MF) of 7 mT on formalde- hyde (FA) biodegradation by activated sludge in synthetic wastewater. The MF had a positive effect on activated sludge biomass growth and dehydrogenase activity. The influence of the MF on the degradation process was observed with a FA concentration of 2400–2880 mg/l. Decreases in FA concentration and chemical oxygen demand (COD) were greater, by 30% and 26% respectively, than those in the control sample. At initial FA concentrations in raw wastewater of 2400 and 2880 mg/l, a decrease in the waste- water biodegradation efficiency was observed. This resulted in an increase of the ecotoxicity of the efflu- ent to Daphnia magna. The value of the sludge biotic index (SBI) was dependent on the FA concentration in raw wastewater and the induction of the MF.