The results of experiments showed an effect of negative magnetic field on some chemical and physical properties of water. A slight increase in iron (Fe) content, a decrease in zinc (Zn) level and an increase in surface tension (Dc) were observed in the study. At the same time the effect of magnetically modified water on an increase in effectiveness of tested zoocides was observed in the control of the two-spotted spider mite (Tetranychus urticae Koch). A strong and statistically significant increase in the effectiveness of zoocide Talstar 100 EC was recorded after the application of one and three magnetizers, preparation Omite 30 WP after the application of two magnetizers, acaricide Magus 200 SC after the application of one and two magnetizers as well as a simultaneous use of one magnetizer and two semi-rings. Acaricide Ortus 05 SC showed a marked increase in the effectiveness in all treatments with the use of magnetically modified water.