The paper presents the results of research on magnesium effect on beneficial enthomopathogenic fungi and nematodes as well as some predatory arthropods. Magnesium fertilization of soil contaminated with heavy metals does not significantly affect numbers of the majority of epigeal invertebrates, but it may influence numbers of single species, favouring the occurrence of some (Bembidion sp.) while restricting the presence of others (Harpalus rufipes De Geer). Magnesium synergism with heavy metal ions has been found to increase infectiveness and pathogenicity of enthomopathogenic fungi. Magnesium, in a dose of 160 mg⋅dm-3 present in the medium, significantly enhances pathogenicity of fungi, such as Beauveria bassiana, Paecilomyces farinosus, Paecilomyces fumoso-roseus or Metarhizium ansopliae. Increased pathogenicity of Steinernema carpocapsae and Heterorhabditis bacteriophora towards test insects has also been observed when magnesium ions were added to a solution in which these nematodes were kept. An effective magnesium dose differed depending whether the nematodes were used separately for test insects (450 mg⋅dm-3) or jointly with enthomopathogenic fungi (320 mg⋅dm-3). This protective effect of magnesium ions on beneficial microorganisms has also been observed in soil contaminated with heavy metals. An addition of magnesium to a solution in which enthomopathogenic nematodes were kept (160 mg⋅dm-3) and to a medium on which fungi were cultured (320 mg⋅dm-3) increased pathogenic abilities of these organisms in contaminated soil to a very high degree (10- to 300- fold higher than the natural heavy metal content in soil).