In this in vitro study, the influence of tin ions at concentrations of 1–1,000 ppm on the development and enzymatic activity of four entomopathogenic fungi (<em>Beauveria bassiana</em>, <em>B. brongniartii</em>, <em>Isaria fumosorosea</em>, and <em>Metarhizium robertsii</em>), that are commonly used in biological plant protection, are examined. Each of the fungal species tested reacted differently to contact with the Sn²⁺ ions at the tested concentrations. Exposure to Sn²⁺ ions affected the rate of development, morphology, and enzymatic activity of fungi. Of the four fungal species studied, <em>M. robertsii</em> was the most resistant and showed complete growth inhibition at the highest Sn²⁺ concentration tested (1,000 ppm). For the other entomopathogenic fungi, the fungicidal effect of Sn²⁺ ions was noted at the concentration of 750 ppm. Exposure to Sn²⁺ ions (up to 500 ppm) resulted in enhanced biochemical activity; and all entomopathogens that were tested showed increased production of <em>N</em>-acetyl-β-glucosaminidase (NAG) as well as several proteases. Moreover, <em>B. brongniartii</em> and <em>M. roberstii</em> showed increased lipases synthesis. These changes may increase the pathogenicity of the fungi, thereby making them more effective in limiting the population of pest insects. The exposure of the entomopathogenic fungi to a medium containing Sn²⁺ ions, at concentrations that were appropriate for each species, induced hyperproduction of hydrolases, which might be involved in aiding the survival of entomopathogenic fungi in the presence of heavy metals. This study shows that the fungistatic effect of Sn²⁺ on entomopathogenic fungi did not restrict their pathogenicity, as evidenced by the stimulation of the production of enzymes that are involved in the infection of insects.