This study was designed to redescribe the cestode Polyonchobothrium clarias that has been recovered from the stomach of Clarias gariepinus, in addition to the ultrastructure of the parasite and the histopathological changes of the
stomach of Clarias gariepinus infected with this parasite. Samples of C.gariepinus were dissected and examined for parasites and prepared for SEM,while the stomach of the host was prepared for histological observations. There
were significant differences between normal fish and infected ones, where the stomach of normal fish exhibited continuous mucosal layer with well developed tubules and ducts with little infiltration of lymphocytes, while a remarkable
damage was observed in the mucosal layer of the stomach of the infected fishwith excessive infiltration of lymphocytes in addition to excessive mucous secretion and subsequent fibrosis.This study has provided further information on
Polyonchobothrium clarias that can reveal more information about host-parasite relationships, particularly its impact on the host fish.