To evaluated the histological changes within the tegument of adult Fasciola gigantica (F. gigantica) that led to the gross changes that were visible externally. Methods: The effects of oleoresin extract of myrrh (Mirazid ? ), myrrh volatile oil and triclabendazole sulphoxide (reference drug) on the tegumental structure of adult F. gigantica following treatment in vitro had been determined by light microscopy. Results: The internal changes in the tegument observed in this study were compatible with surface changes seen in the previous scanning electron microscopy study, using the same drugs. The swelling of tegumental syncytium was a particular feature of their action, but its level was much greater with myrrh volatile oil, in which vacuolization of the tegument and loss of spines were observed. Conclusions: The present study demonstrated the fasciocidal properties of Mirazid ? oleoresin extract, and it might be possible to reinforce its fasciocidal activity by increasing its content of myrrh volatile oil.