In the recent years, a high mortality of bee colonies is reported (colony collapse disorder). Varroa mites (Varroa destructor) are one of the biggest threats in beekeeping. Beekeepers use a wide range of different synthetic acaricides to keep varroa mites under control. The use of natural acaricides is also increased. Among them the essential oils and their active components are known, including thymol. The aim of this master's thesis was to examine the effect of thymol on enzyme acetycholineesterase (AChE). The main goal was to determine, if the usage of thymol in acaricidal purposes is safe for bees. We were also monitoring the activity of antioxidant enzymes glutathione S-transferase (GST). The bees were chronically exposed for 7 days to different concentrations of thymol (0 %, 0,05 %, 0,5 % in 1 %; mass in the bee food). At the end of the experiment, we determined the activity of AChE and GST in the heads and thoraxes of bees. We found out that AChE activity in heads increased at 0,5 % in 1 % of thymol and GST activity in heads and thoraxes increased at all three tested concentrations (0,05, 0,5 in 1 %). We also found out that during the experiment, the feeding rate decreased, but also in control group therefore the decrease was not induced by thymol. Our study showed that the highest tested concentration (1 %) is not safe for bees, as it causes bee mortality. At lower concentrations, changes in the activities of enzymes associated with the transmission of the nervous system and detoxification processes were shown.