Some pollutants, in normal atmospheric conditions, are removed quickly from environment, because they serve as a source of nutrition and energy for active microorganisms. Soil microorganisms, due to their metabolic functions, can use pesticides, herbicides as a source of carbon and energy. The effect of chemical substances depends on the variety of microorganisms, their form and number. Microorganisms are sensitive to a chemical when they are in vegetative state and more resistant in sporulated state. Young cells, with a higher content of free water are more susceptible than mature cells. In the case of high microbial load, the chemical efficiency is reduced. The aim of the research was to study the ability of fungal strains to microbiological transformation of trifluralin in the presence of Fe3O4 nanoparticles. For the research were used 50 fungal strains stored in the National Collection of Non- Pathogenic Microorganisms. There were tested five concentrations of trifluralin (TF) (mg /L): 50; 100; 200; 300 and 500 and Fe3O4 nanoparticles (NP) of 3 sizes (17–20 nm, 20–25 nm and 50–70 nm) in 5 concentrations (mg /L): 1; 10; 25; 50; 100. As control sample served medium Czapek without glucose. Growing strains on the agar medium supplemented with different doses of trifluralin, we noticed that all tested strains, even those that do not grow on Czapek medium without glucose, grow and develop very well, regardless of trifluralin concentration in the agar medium. Trifluralin in this case acted as a stimulator for micromycetes, or as an additional source of carbon. To study the action of nanoparticles on micromycetes capable to growth in the presence of trifluralin, were selected 8 strains, 4 capable to grow on Czapek medium without glucose and 4 not capable to grow on this medium. The selected strains were cultivated on Czapek medium without glucose + 500 ml trifluralin in the presence of various concentrations of Fe3O4 nanoparticles. After monitoring the microbial growth, we found 2 strains unable to grow, 3 strains registered the same parameters as on the medium with trifluralin and in the case of 3 strains (Penicillium sp.11, Trichoderma sp.5 and Trichoderma sp.15) the growth was stimulated by Fe3O4 nanoparticles We found that the action of NP on micromycetes grown in presence of TF depends on the culture medium (agar or liquid), concentration and size of NP. Nanoparticles acted more efficiently by small size – 17–20 nm and 20–25 nm, both on growth and development of fungal strains as well as on the biosynthetic activity. The optimum concentration of the nutrient medium supplemented with NP was 1–25 mg/L. The increase of the concentration led to decrease of NP effect, which in high doses may inhibit the growth of cultures. Most remarkable results were registered in the case of submerged cultivation of Trichoderma sp.5 on the medium supplemented with 10–25 mg/L Fe3O4NP (50–70 nm) and 300 mg/L trifluralin, as well as on the medium containing 500mg/L TF +10 mg/L and 100 mg/L Fe3O4NP (50–70 nm). In absence of trifluralin, the studied strains were found to grow at low concentrations of NP – 1–10 mg/L regardless of their size. According to the results, we can conclude that the Fe3O4 NP reduce the toxicity of trifluralin and stimulate the growth and biosynthetic activity of micromycetes.