The study of different species of tunas has revealed high numbers of cysts of Koellikerioides internogastricus Yamaguti, 1970, parasitizing Thunnus atlanticus (Lesson), Thunnus albacares (Bonnaterre) and Thunnus obesus (Lowe), caught along the Rio de Janeiro coastline, Atlantic Ocean, Brazil. This parasite was isolated from mucosa and submucosa of the stomach of the three tuna species studied. We report K. internogastricus for the first time on the Brazilian coast, representing the first record of this species in South America and in T. atlanticus, a new host. The pathological alterations induced by K. internogastricus in the stomach of T. albacares, T. atlanticus and T. obesus are also described for the first time. Original measurements and figures are presented.