Microplastics pollution of the environment is a global problem that was not realized at the very beginning of the plastic materials mass production, but today we are finding more and more negative effects on terrestrial, marine and freshwater organisms. Textile plastic microfibers (so-called microfibers, length bellow 5 mm) are one of many microplastic forms, accumulating in the environment. With the master master`s thesis we wanted to check the effect of microfibers (length 15–1288 µm) and textile plastic mesofibers (so-called mesofibers, 11.81 ± 3.7 mm) on the tererestrial isopod Porcellio scaber Latreille, after prolonged four week exposure. We monitored feeding, growth and level of the energy reserves (carbohydrates, lipids, proteins) of terrestrial isopods exposed to micro/mesofibers in the soil and the occurrance of the the selective behaviour of isopods, exposed to uncontaminated and with microfibers contaminated soil. We found out that chronic exposure to the selected concentrations of microfibers and mesofibers in the soil, does not have a statistically significant effect on the selected parameters of the terrestrial isopods. It would be important to examine the impact of the microplastic accumulation in the environment at even higher concentrations and after a longer chronic exposure and to use additional methods for monitoring labaratory measurable sublethal responses of experimental organisms.