Abstract: A new trematode genus Thynstenopera was described here from the intestine of the fish Thynnus thunnia of Karachi coast, in the family Opecoeliidae and subfamily Plagioporinae. The genus is characterized by having flattened body with distinct demarcation of the forebody which is very short. Oral sucker is small, pharynx not visible, caeca long, narrow, terminating near posterior extremity of the body. Acetabulum larger than oral sucker, near anterior extremity. Testes tandem, close together, deeply lobed, postequatorial. Cirrus pouch is curved, very long reaching far posterior to acetabulum. Genital pore slightly submedian at the level of esophageal bifurcation. Ovary lobed, pretesticular and close to it, seminal receptacle lateral to ovary, Laurer`s canal present. Vitellaria prominent in lateral fields of the hind body confluent in the testicular and post testicular region. Uterus was in diagonal spirals between ovary and acetabulum. Eggs relatively large without polar filaments. Species name T. lobata is proposed because of lobed ovary and testes.