Crustaceans Daphnia magna Straus (1820) are used in many countries to assess the quality of the environment. Researchers are guided more often by mortality of individuals. However, sublethal effects contribute to the development of biological early warning systems (BEWS). We proposed a visual method for recording the motor activity of D. magna. This method has advantages over automated analogs. It is simple, accessible to performers and does not require the use of special instruments. We tested the method under conditions of modeling the lethal and sublethal effects of heavy metal salts (Cu and Zn). Diagnosis of lethal doses is possible after 1 hour of exposure, whereas the death of crustaceans can occur in 3-4 days of the experiment. The effect of sublethal doses of heavy metals becomes statistically significant after 24 (p <0.05). The sensitivity of the method was confirmed in studies of aqueous extracts from soils contaminated with various heavy metals (heavy metal processing area). The method showed good results in assessing the toxic effect of waters anthropogenically contaminated with mineral forms of nitrogen (NH4+и NO3-) in the region of fertilizer production. The maximum inverse correlation is shown after 24 hours between the motor activity and the concentration of ammonium ions in water (-0.83). Thus, the method of visual assessment of motor activity can be used in environmental monitoring.