In recent decades the use of nanotechnologies in the remediation of xenobiotic substances has proven its effectiveness, but not its safety. Nanoparticles often accumulate in the remedied environment, having, over time, toxic effects on living organisms. In this context, research on the vital activity of microorganisms and their interaction with nanoparticles is of major importance. Aim of the research was to determine the influence of Fe₃ O₄ nanoparticles, obtained by different ways (laboratory method and synthesis in the reactor) on the viability and colony morphology of Rhodococcus rhodochrous CNMN-Ac-05 strain. Methods. Encapsulated magnetite (Fe₃ O₄ ) nanoparticles were synthesized by chemical co-precipitation method, using iron(II) sulfate and iron(III) chloride in the presence of poly-N-vinylpyrrolidone, used as a stabilizer. Fe₃ O₄ SR (Synthesis in the Reactor) was produced in the multifunctional reactor VGR-50, at the same conditions. Cell biomass was determined on the spectrophotometer by the optical density at 540 nm, with subsequent recalculation to cell dry weight according to the calibration curve. The cell dry weight was determined by gravimetric method. The morphological features of the rhodococci colonies were described according to the standard microbiological method. Results. It was established that magnetite nanoparticles in concentrations of 1–100 mg/L were not toxic to the R. rhodochrous strain, had a positive effect on the viability of rhodococci by stimulating the growth of biomass, regardless of their concentration and the method of their synthesis. In the presence of Fe₃ O₄ nanoparticles the population dissociated to S1, S2, R1, R2 forms, and S-R type of colonies, while the basic morphological features of R. rhodochrous colonies corresponded to type S1. Conclusions. The optimal concentration of magnetite nanoparticles, which stimulated the growth and development of R. rhodochrous was 25 mg/L for Fe₃ O₄ and 50 mg/L Fe₃ O₄ SR. At all concentration of Fe₃ O₄ nanoparticles the main colony morphotype of the rhodococci was smooth S1-type; the new types of colonies represented only 0.1–0.6 % of the population, and the lowest degree of variability corresponded with the highest colony-forming units index.