The object of the study was the effect of metal ions Hg²⁺, Cu²⁺, Zn²⁺ on the development of neustonic and planktonic bacteria from Lake Jeziorak Maly. It was found that planktonic bacteria were more susceptible to the heavy metal ions used in the experiment than neustonic bacteria. The bacteria showed higher tolerance to various concentrations of heavy metals in spring and summer than in autumn and winter. Neustonic bacteria isolated from sites exposed to municipal and industrial pollution showed higher tolerance to increased concentrations of heavy metals. Bacteria cultured in trophically rich medium were less susceptible to various concentrations of heavy metals than those cultured in trophically poor medium. The effect of toxicity of heavy metal ions on bacteria increased with the increase in their concentration and decreased in the following succession: Hg > Cu > Zn.