In the master's thesis, we assessed the parasitism rates of the chestnut gall wasp with the non-native species Torymus sinensis. The parasitism rates were estimated in 2017 and compared with the rates from years 2015 and 2016. In 2017 the galls were collected and dissected and larval chambers, larvae of T. sinensis, D. kuriphilus larvae and pupae in galls were counted. Parasitism rate was determined in a sample of 100 galls where this was possible. Parasitism rate from 2015 to 2017 was rather high. Compared to 2015 parasitism in 2017 slightly increases. In Ravnica it increased from 83,5 % (2015) to 84,9 % (2017), in Zg. Pohanca increased from 79,4 % (2015) to 80,3 % (2017). The parasitism rate in forests is slightly higher than in the orchard. We also investigated the genetic diversity of parasitoids. By analyzing mtCOI the genetic variability of parasitoids T. sinensis was determined according to the demographic distribution, and the diversity of parasitoid from 2015 to 2017 at the same location. In 2017 9 different haplotypes were found. The most common haplotype was HT1, followed by HT2 and other haplotypes (HT3 – HT9). Between 2015 and 2017 genetic changes can be observed at all three sites. According to the findings, it can be concluded that the introduction of non-native species T. sinensis represents a successful example of biological agent.