Fascioloidosis is parasitic disease caused by the big American liver fluke (Fascioloides magna). Invasive disease was recorded for the first time in Italy, near Turin, and until today it is present in Germany, Austria, Slovenia, Hungary, Slovakia and Czech Republic. Invasion of the fallow deer was found in Russia and on the border of Germany and Poland. By migration of invaded deer, big American liver fluke came to Croatia and it was determinate in the East Baranja, for the first time in 1999. The disease was suspected by forest workers and hunters, when they noticed changes in condition, and after necropsy they also found specific lesions in the liver that confirmed the suspicion and systematic treatment of fascioloidosis began. The area of research and determination of post treatment status of fallow deer is game farm no. XVI/8 “Kunjevci”. After shooting, liver are exenterated and examined and it was determined number and development stages of liver fluke Fascioloides magna as well as stage of pathoanatomical changes in liver tissue. Based on qualitative and quantitative examination and comparing the results of three hunting seasons it was evaluated the character of invasion of the deer population and efficiency of the therapy that was carried out. Oral anthelmintic therapy was performed before shooting and sampling. In the 2016/2017 hunting season 40 liver samples were collected, in the 2017/2018 hunting season 25 liver samples and in the 2018/2019 16 liver samples. The results of pathoanatomical and parasitological examination are presented in the form of tables and charts for each season for three consecutive seasons (2016/2017, 2017/2018 and 2018/2019) and the data are arranged by age and gender. From the presented results, we can conclude that the applied method of fascioloidosis treatment of fallow deer in fenced hunting ground gives satisfying success especially among young and mature deer. Continuous exposure of the treated mixture induce hierarchical domination on the feeding. Exposure of the treated mixture in the spring provides protection against re – invasion to dominant males. By late – spring exposure of the treated mixture further reduction of parasitic habitat load is achieved as well as avoiding superinvasion of middle – aged and mature deer. It also benefits the treatment of male that do not take winter feed. Middle – aged deer are extremely suitable to the applied strategy of fascioloidosis treatment.